Skip to main content

Full text of "The origin of species"

See other formats

%>: \ LIB 












Copyright, 1909 
By p. F. Collier & Son 

"But with regard to the material world, we can at 
least go so far as this — we can perceive tliat events are 
brought about not by insulated interpositions of Divine 
power, exerted in each particular case, but by the estab- 
lishment of general laws." 

VViiEWELL: Bridgewnter Treatise. 

"The only distinct meaning of the word 'natural' is 
stated, fixed or settled; since what is natural as much 
requires and presupposes an intelligent agent to render 
it so, i.e., to effect it continually or at stated times, as 
what is supernatural or miraculous does to effect it for 

Butler : Analogy of Revealed Religion. 

"To conclude, therefore, let no man out of a weak con- 
ceit of sobriety, or an ill-applied moderation, think or 
maintain, that a man can search too far or be too well 
studied in the book of God's word, or in the book of 
God's works; divinity or philosophy: but rather let men 
endeavour an endless progress or proficience in both." 
Bacon: Advancement of Learning. 

Down, Beckenham, Kent, 

First Edition, 'November 2^th, 1859. 
Sixth Edition, Jamuary, 18112. 






Of the Progress of Opinion on the Origin of Species 9 


Variation under Domestication 25 

Variation under Nature 58 

Struggle for Existence 76 

Natural Selection; or the Survival op the Fittest 93 

Laws of Variation ^45 

Difficulties of the Theory i/S 



Miscellaneous Objections to the Theory of Natural 
Selection 219 

Instinct 262 

Hybridism 298 

On the Imperfection of the Geological Record . 353 

On the Geological Succession of Organic Beinss , 364 

Geographical Distribution 395 

Geographical Distribution — continued .... 427 


Mutual Affinities of Organic Beings: Morphology: 
Embryology: Rudimentary Organs .... 450 

Recapitulation and Conclusion 499 


INDEX 541 


Charles Robert Darwin, horn at Shrcii'sbiiry, England, on 
February 12, 1809, came of a family of remarkable intellectual 
distinction which is still sustained in the present generation. His 
father tvas a successful physician zvith remarkable powers of 
observation, and his grandfather was Erasmus Danvin, the well- 
knozvn author of "The Botanic Garden." He went to school at 
Shrewsbury, zvhere he failed to profit from the strict classical 
curriculum there in force; nor did the regular professional 
courses at Edinburgh University, zvhere he spent two years study- 
ing medicine, succeed in rousing his interest. In 1827 he was 
entered at Christ's College, Cambridge, to study for the B. A. 
degree, preparatory to entering the Church; hut zvhile there 
his friendship with Henslow, the professor of botany, led to his 
enlarging his general scientific knozvledge and finally to his 
joining the expedition of the "Beagle" in the capacity of natural- 
ist. From this Darzmn returned after a voyage of five years 
with a vast first-hand knozvledge of geology and zoology, a 
reputation as a successful collector, and, most important of all, 
with the germinal ideas of his theory of evolution. The next 
few years were spent in working up the materials he had col- 
lected; hut his health gave signs of breaking, and for the rest 
of his life he suffered constantly, but without complaint. With 
extraordinary courage and endurance he took up a life of 
seclusion and methodical regularity, and accomplished his colossal 
results in spite of the most severe physical handicap. He had 
married in 1839, and three years later he withdrczv from London 
to the little village of Dozvn, about sixteen miles out, zvhere he 
spent the rest of his life. His custom, zvhich zvas almost a 
method, was to work till he was on the verge of complete collapse, 
and then to take a holiday just sufficient to restore him to zvorking 

As early as 1842 Darwin had thrown into rough form the out- 
lines of his theory of evolution, hut the enormous extent of the 
inve'stigations he engaged in for the purpose of testing it led 
to a constant postponing of publication. Finally in June, 1S5S, 
A. R. Wallace sent him a manuscript containing a statement 
of an identical theory of the origin of species, which had been 



arrived at entirely independently. On the advice of Lyell, the 
geologist, and Hooker, the botanist, Wallace's paper and a letter 
of Darivin's of the previous year, in ivhich he had outlined his 
theory to Asa Gray, zvere read together on July I, 1858, and 
published by the Linncean Society. In November of the follow- 
ing year "The Origin of Species" was published, and the great 
battle was begun between the old science and the new. This 
work was followed in 1868 by his "Variation of Animals and 
Plants under Domestication," that in turn by the "Descent of 
Man" in i8yi, and that again by "The Expression of the Emo- 
tions in Man and Animals." Each of these books was the elabo- 
ration or complement of a section of its predecessor. The later 
years of Darwin's life were chieAy devoted to botanical research, 
and resulted in a series of treatises of the highest scientific value. 
He died at Down on April ig, 1882, and is buried in Westminster 

The idea of the evolution of organisms, so far from originating 
with Darwin, is a very old one. Glimpses of it appear in the 
ancient Greek philosophers, especially Empedocles and Aristotle; 
modern philosophy from Bacon onward shows an increasing 
definiteness in its grasp of the conception; and in the age pre- 
ceding Darivin's, Buffon, Erasmus Darwin, and Lamarck had 
given it a fairly concrete expression. As we approach the date 
of the publication of "The Origin of Species" adherence to the 
doctrine not only by naturalists but by poets, such as Goethe, 
becomes comparatively frequent; and in the six years before the 
joint announcement of Darwin and Wallace, Herbert Spencer 
had been supporting and applying it vigorously in the field of 

To these partial anticipations, however, Darwin oiued little. 
When he became interested in the problem, the doctrine of the 
fixity of species was still generally held; and his solution occurred 
to him mainly as the result of his own observation and thinking. 
Speaking of the voyage of the "Beagle," he says, "On my return 
home in the autumn of 1836 I immediately began to prepare my 
journal for publication, and then saw how many facts indicated 
the common descent of species. ... In July (1837) I opened 
my first note-book for facts in relation to the Origin of Species, 
about which I had long reflected, and never ceased working for 
the next twenty years. . . . Had been greatly struck from about 


the month of previous March on character of South American 
fossils, and species on Galapagos Archipelago. These facts 
(especially latter) origin of all my views." Again, "In October 
(1838), that vs fifteen months after I had begun my systematic 
inquiry, I happened tr> read for amusement 'Malthus on Popu- 
lation,' and being well prepared to appreciate the struggle for 
existence which everywhere goes on from long-continued ob- 
servation of the habits of animals and plants, it at once struck me 
that under these circumstances favorable variations would tend 
to be preserved, and unfavorable ones to be destroyed. The 
result of this zvould be the formation of new species. Here 
then I had at laist got a theory by which to work." 

From these statements by Darwin himself we can see how far 
it is from being the case that he merely gathered the ripe fruit 
of the labors of his predecessors. All progress is continuous, 
and Darwin, like other men, built on the foundations laid by 
others; but to say this is not to deny him originality in the only 
vital -sense of that word. And the importance of his contribution 
— in verifying the doctrine of descent, in interpreting and apply- 
ing it, and in revealing its bearings on all departments of the 
investigation of nature — is proved by the fact that his work 
opened a new epoch in science and philosophy. As Huxley said, 
"Whatever be the ultimate verdict of posterity upon this or that 
opinion which Mr. Darwin has propounded; whatever adumbra- 
tions or anticipations of his doctrines may be found in the writ- 
ings of his predecessors ; the broad fact remains that, since the 
publication and by reason of the publication of 'The Origin of 
Species' the fundamental conceptions and the aims of the students 
of living Nature have been completely changed." 

The present year (1909) has seen the celebration of the hun- 
dredth anniversary of Darwin's birth and the fiftieth anniversary 
of the publication of his great work. Among the numerous ex- 
pressions of honor and gratitude which the world of science has 
poured upon his memory, none is more significant than the vol- 
ume on "Darwin and Modern Science" which has been issued by 
the press of his old University of Cambridge. In this are col- 
lected nearly thirty papers by the leaders of modern science 
dealing with the influence of Darwin upon various fields of 
thought and research, and with the later developments and modi- 
fications of his conclusions. Biology, in many different depart- 


ments. Anthropology, Geology, Psychology, Philosophy, Soci- 
ology, Religion, Language, History, and Astronomy are all repre- 
sented, and the mere enumeration suggests the colossal nature 
of his achievement and its results. 

Yet the spirit of the man was almost as wonderful as his work. 
His disinterestedness, his modesty, and his absolute fairness 
were not only beautiful in themselves, but remain as a proof of 
the importance of character in intellectual labor. Here is his 
own frank and candid summing up of his abilities: "My success as 
a man of science, whatever this may have amounted to, has been 
determined, as far as I can judge, by complex and diversified 
mental qualities and conditions. Of these, the most important 
have been — the love of science — unbounded patience in long re- 
flecting over any subject — industry in observing and collecting 
facts — and a fair share of invention as zvell as of common sense. 
With such moderate abilities as I possess, it is truly surprising 
that I should have influenced to a considerable extent the belief 
of scientific men on some important points." 




I WILL here give a brief sketch of the progress of opinion on 
the Origin of Species. Until recently the great majority of 
naturalists believed that species were immutable productions, 
and had been separately created. This view has been ably 
maintained by many authors. Some few naturalists, on the 
other hand, have believed that species undergo modification, 
and that the existing forms of life are the descendants by 
true generation of pre-existing forms. Passing over allu- 
sions to the subject in the classical writers,* the first author 
who in modern times has treated it in a scientific spirit was 
Buffon. But as his opinions fluctuated greatly at different 
periods, and as he does not enter on the causes or means of 
the transformation of species, I need not here enter on 

*Aristotle, in his ' Physicae Auscultationes ' (lib. 2, cap. 8, s. 2). after 
remarking that rain does not fall in order to make the corn grow, any more 
than it falls to spoil the farmer's corn when threshed out of doors, applies 
the same argument to organisation; and adds (as translated by Mr. Clair 
Grace, who first pointed out the passage to mc), "So what hinders the dif- 
ferent parts [of the body] from having this merely accidental relation in 
nature? as the teeth, for example, grow by necessity, the front ones sharp, 
adapted for dividing, and the grinders flat, and serviceable for masticating 
the food; since they were not made for the sake of this, but it was the 
result of accident. And in like manner as to the other parts in which there 
appears to exist an adaptation to an end. Wheresoever, therefore, all things 
together (that is all the parts of one whole) happened like as if they were 
made for the sake of something, these were preserved, having been appro- 
priately constituted by an internal spontaneity; and whatsoever things w;ere 
not thus constituted, perished, and still perish." We here sec tiie principle 
of natural selection shadowed forth, but how little Aristotle fully compre- 
hended the principle, is shown by his remarks on the formation of the teeth. 



Lamarck was the first man whose conclusions on the 
subject excited much attention. This justly-celebrated nat- 
uralist first published his views in 1801 ; he much enlarged 
them in 1809 in his 'Philosophic Zoologique,' and subse- 
quently, in 1815, in the Introduction to his 'Hist. Nat. des 
Animaux sans Vertebres.' In these works he upholds the 
doctrine that species, including man, are descended from 
other species. He first did the eminent service of arousing at- 
tention to the probability of all change in the organic, as well 
as in the inorganic world, being the result of law, and not of 
miraculous interposition. Lamarck seems to have been chiefly 
led to his conclusion on the gradual change of species, by the 
difficulty of distinguishing species and varieties, by the almost 
perfect gradation of forms in certain groups, and by the 
analogy of domestic productions. With respect to the means 
of modification, he attributed something to the direct action 
of the physical conditions of life, something to the crossing of 
already existing forms, and much to use and disuse, that is, 
to the efifects of habit. To this latter agency he seems to 
attribute all the beautiful adaptations in nature; — such as the 
long neck of the giraffe for browsing on the branches of 
trees. But he likewise believed in a law of progressive de- 
velopment; and as all the forms of life thus tend to progress, 
in order to account for the existence at the present day of 
simple productions, he maintains that such forms are now 
spontaneously generated.* 

Geoffroy Saint-Hilaire, as is stated in his 'Life,' written 
by his son, suspected, as early as 1795, that what we call 
species are various degenerations of the same type. It was 

* I have taken the date of the first publication of Lamarck from Isid. 
Geoffroy Saint-Hilaire]s ('Hist. Nat. Generale,' torn, ii, p. 405, 1859) 
excellent history of opinion on this subject. In this work a full account is 
given of Buffon's conclusions on the same subject. It is curious how largely 
my grandfather, Dr. Erasmus Darwin, anticipated the views and erroneous 
grounds of opinion of Lamarck in his ' Zoonomia ' (vol. i. pp. 500-510), pub- 
lished in 1794. According to Isid. Geoffroy there is no doubt that Goethe 
was an extreme partisan of similar views, as shown in the Introduction to a 
work written in 1794 and 1795, but not published till long afterwards: he 
has pointedly remarked (' Goethe als Naturforscher,' von Dr. Karl Meding, 
s. 34) that the future question for naturalists will be how, for instance, 
cattle got their horns, and not for what they are used. It is rather a singu- 
lar instance of the manner in which similar views arise at about the same 
time, that Goethe in Germany, Dr. Darwin in England, and Geoffroy Saint- 
Hilaire (as we shall immediately see) in France, came to the same conclu- 
sion on the origin of species, in the years 1794-5. 


not until 1828 that he published his conviction that the same 
forms have not been perpetuated since the origin of all 
things. Geoffroy seems to have relied chiefly on the condi- 
tion of life, or the "monde ambiant" as the cause of change. 
He was cautious in drawing conclusions, and did not believe 
that existing species are now undergoing modification; and, 
as his son adds, "C'est done un probleme a reserver 
entierement a I'avenir, suppose meme que I'avenir doive avoir 
prise sur lui." 

In 1813, Dr. W. C. Wells read before the Royal Society 
'An Account of a White female, part of whose skin re- 
sembles that of a Negro' ; but his paper was not published 
until his famous * Two Essays upon Dew and Single Vision' 
appeared in 1818. In this paper he distinctly recognises the 
principle of natural selection, and this is the first recognition 
which has been indicated ; but he applies it only to the races of 
man, and to certain characters alone. After remarking that 
negroes and mulattoes enjoy an immunity from certain trop- 
ical diseases, he observes, firstly, that all animals tend to vary 
in some degree, and, secondly, that agriculturists improve 
their domesticated animals by selection ; and then, he adds, 
but what is done in this latter case ''by art, seems to be done 
with equal efficacy, though more slowly, by nature, in the 
formation of varieties of mankind, fitted for the country 
which they inhabit. Of the accidental varieties of man, 
which would occur among the first few and scattered inhab- 
itants of the middle regions of Africa, some one would be 
better fitted than the others to bear the diseases of the coun- 
try. This race would consequently multiply, while the others 
would decrease; not only from their inability to sustain the 
attacks of disease, but from their incapacity of contending 
with their more vigorous neighbours. The colour of this 
vigorous race I take for granted, from what has been already 
said, would be dark. But the same disposition to form varie- 
ties still existing, a darker and a darker race would in the 
course of time occur : and as the darkest would be the best 
fitted for the climate, this would at length become the most 
prevalent, if not the only race, in the particular country in 
which it had originated." He then extends these same views 
to the white inhabitants of colder climates. I am indebted 


to Mr. Rowley, of the United States, for having called my 
attention, through Mr. Brace, to the above passage in Dr. 
Wells' work. 

The Hon. and Rev. W. Herbert, afterwards Dean of Man- 
chester, in the fourth volume of the ' Horticultural Trans- 
actions,' 1822, and in his work of the 'Amaryllidacese' 
(1837, pp. 19, 339), declares that "horticultural experiments 
have established, beyond the possibility of refutation, that 
botanical species are only a higher and more permanent class 
of varieties." He extends the same view to animals. The 
Dean believes that single species of each genus were created 
in an originally highly plastic condition, and that these have 
produced, chiefly by intercrossing, but likewise by variation, 
all our existing species. 

In 1826 Professor Grant, in the concluding paragraph in 
his well-known paper ('Edinburgh Philosophical Journal,' 
vol. xiv. p. 283) on the Spongilla, clearly declares his belief 
that species are descended from other species, and that they 
become improved in the course of modification. This same 
view was given in his 55th Lecture, published in the ' Lancet ' 
in 1834. 

In 1831 Mr. Patrick Matthew published his work on 'Naval 
Timber and Arboriculture,' in which he gives precisely the 
same view on the origin of species as that (presently to be 
alluded to) propounded by Mr. Wallace and myself in the 
'Linnean Journal,' and as that enlarged in the present volume. 
Unfortunately the view was given by Mr. Matthew very brief- 
ly in scattered passages in an Appendix to a work on a differ- 
ent subject, so that it remained unnoticed until Mr. Matthew 
himself drew attention to it in the 'Gardener's Chronicle,' on 
April 7th, i860. The differences of Mr. Matthew's view from 
mine are not of much importance: he seems to consider that 
the world was nearly depopulated at successive periods, and 
then re-stocked; and he gives as an alternative, that new 
forms may be generated " without the presence of any mould 
or germ of former aggregates." I am not sure that I under- 
stand some passages; but it seems that he attributes much 
influence to the direct action of the conditions of life. He 
clearly saw, however, the full force of the principle of natural 


The celebrated geologist and naturalist, Von Buch', in his 
excellent 'Description Physique des Isles Canaries' (1836, 
p. 147), clearly expresses his belief that varieties slowly be- 
come changed into permanent species, which arc no longer 
capable of intercrossing. 

Rafinesque, in his 'New Flora of North America,' pub- 
lished in 1836, wrote (p. 6) as follows : — " All species might 
have been varieties once, and many varieties are gradually 
becoming species by assuming constant and peculiar charac- 
ters;" but farther on (p. 18) he adds, "except the original 
types or ancestors of the genus." 

In 1843-44 Professor Haldeman (' Boston Journal of Nat. 
Hist. U. States,' vol. iv. p. 468) has ably given the arguments 
for and against the hypothesis of the development and modi- 
fication of species : he seems to lean towards the side of 

The 'Vestiges of Creation' appeared in 1844. In the tenth 
and much improved edition (1853) the anonymous author 
says (p. 155) : — " The proposition determined on after much 
consideration is, that the several series of animated beings, 
from the simplest and oldest up to the highest and most re- 
cent, are, under the providence of God, the results, first, of an 
impulse which has been imparted to the forms of life, ad- 
vancing them, in definite times, by generation, through grades 
of organisation terminating in the highest dicotyledons and 
vertebrata, these grades being few in number, and generally 
marked by intervals of organic character, which we find to 
be a practical difficulty in ascertaining affinities ; second, of 
another impulse connected with the vital forces, tending, in 
the course of generations, to modify organic structures in 
accordance with external circumstances, as food, the nature 
of the habitat, and the meteoric agencies, these being the 
' adaptations ' of the natural theologian." The author ap- 
parently believes that organisation progresses by sudden 
leaps, but that the effects produced by the conditions of life 
are gradual. He argues with much force on general grounds 
that species are not immutable productions. But I cannot see 
how the two supposed " impulses " account in a scientific 
sense for the numerous and beautiful co-adaptations which 
we see throughout nature; I cannot see that we thus gain 


any insight how, for instance, a woodpecker has become 
adapted to its peculiar habits of life. The work, from its 
powerful and brilliant style, though displaying in the earlier 
editions little accurate knowledge and a great want of 
scientific caution, immediately had a very wide circulation. 
In my opinion it has done excellent service in this country in 
calling attention to the subject, in removing prejudice, and 
in thus preparing the ground for the reception of analogous 

In 1846 the veteran geologist M. J. d'Omalius d'Halloy 
published in an excellent though short paper ('Bulletins de 
I'Acad. Roy. Bruxelles,' torn. xiii. p. 581) his opinion that 
it is more probable that new species have been produced by 
descent with modification than that they have been separately 
created: the author first promulgated this opinion in 1831. 

Professor Owen, in 1849 ('Nature of Limbs,' p. 86), 
wrote as follows: — "The archetypal idea was manifested in 
the flesh under diverse such modifications, upon this planet, 
long prior to the existence of those animal species that 
actually exemplify it. To what natural laws or secondary 
causes the orderly succession and progression of such organic 
phenomena may have been committed, we, as yet, are igno- 
rant." In his Address to the British Association, in 1858, 
he speaks (p. li.) of "the axiom of the continuous operation 
of creative power, or of the ordained becoming of living 
things." Farther on (p. xc), after referring to geographical 
distribution, he adds, "These phenomena shake our confi- 
dence in the conclusion that the Apteryx of New Zealand 
and the Red Grouse of England were distinct creations in and 
for those islands respectively. Always, also, it may be well 
to bear in mind that by the word ' creation ' the zoologist 
means ' a process he knows not what' " He amplifies this 
idea by adding that when such cases as that of the Red 
Grouse are " enumerated by the zoologist as evidence of dis- 
tinct creation of the bird in and for such islands, he chiefly 
expresses that he knows not how the Red Grouse came to be 
there, and there exclusively ; signifying also, by this mode of 
expressing such ignorance, his belief that both the bird and 
the islands owed their origin to a great first Creative Cause." 
If we interpret these sentences given in the same Address, 


one by the other, it appears that this eminent philosopher felt 
in 1858 his confidence shaken that the Apteryx and the Red 
(Grouse first appeared in their respective homes, "he knew 
not how," or by some process "he knew not what." 

This Address was delivered after the papers by Mr. Wal- 
lace and myself on the Origin of Species, presently to be 
referred to, had been read before the Linnean Society. When 
the first edition of this work was published, I was so com- 
pletely deceived, as were many others, by such expressions as 
" the continuous operation of creative power," that I included 
Professor Owen with other palaeontologists as being firmly 
convinced of the immutability of species ; but it appears 
(' Anat. of Vertebrates,' vol. iii. p. 796) that this was on my 
part a preposterous error. In the last edition of this work 
I inferred, and the inference still seems to me perfectly just, 
from a passage beginning with the words " no doubt the type- 
form," &r. (Ibid. vol. i. p. XXXV.). that Professor Owen 
admitted that natural selection may have done something in 
the formation of a new species; but this it appears (Ibid. vol. 
iii. p. 798) is inaccurate and without evidence. I also gave 
some extracts from a correspondence between Professor 
Owen and the Editor of the * London Review,' from which it 
appeared manifest to the Editor as well as to myself, that 
Professor Owen claimed to have promulgated the theory of 
natural selection before I had done so; and I expressed my 
surprise and satisfaction at this announcement ; but as far 
as it is possible to understand certain recently published pas- 
sages (Ibid. vol. iii. p. 798) I have either partially or wholly 
again fallen into error. It is consolatory to me that others 
find Professor Owen's controversial writings as difficult to 
understand and to reconcile with each other, as I do. As far 
as the mere enunciation of the principle of natural selection 
is concerned, it is quite immaterial whether or not Professor 
Owen preceded me, for both of us, as shown in this histori- 
cal sketch, were long ago preceded by Dr. Wells and Mr. 

M. Isidore Geofifroy Saint-Hilaire, in his lectures delivered 
in 1850 (of which a Resume appeared in the 'Revue et Mag. 
de Zoolog.,' Jan. 1851), briefly gives his reason for believing 
that specific characters "sont fixes, pour chaque cspcce, tant 


qu'elle se perpetue ou milieu des memes circonstances : ils se 
modifient, si les circonstances ambiantes viennent a changer." 
"En resume, I'cbservaiion des animaux sauvages demontre 
deja la variabilite limitee des especes. Les experiences sur 
les animaux sauvages devenus domestiques, et sur les ani- 
maux domestiques redevenus sauvages, la demontrent plus 
clairement encore. Ces memes experiences prouvent, de 
plus, que les dififerences produites peuvent etre de valeur 
generique." In his ' Hist. Nat. Generale' (torn ii. p. 340, 
1859) he amplifies analogous conclusions. 

From a circular lately issued it appears that Dr. Freke, in 
1851 ('Dublin Medical Press,' p. 322), propounded the doc- 
trine that all organic beings have descended from one pri- 
mordial form. His grounds of belief and treatment of the 
subject are wholly different from mine; but as Dr. Freke 
has now^ (1861) published his Essay on the 'Origin of Spe- 
cies by means of Organic Affinity,' the difficult attempt to 
give any idea of his views would be superfluous on my part. 

Mr. Herbert Spencer, in an Essay (originally published in 
the 'Leader,' March, 1852, and republished in his 'Essays,' in 
1858), has contrasted the theories of the Creation and the 
Development of organic beings with remarkable skill and 
force. He argues from the analogy of domestic productions, 
from the changes which the embryos of many species under- 
go, from the difficulty of distinguishing species and varie- 
ties, and from the principle of general gradation, that species 
have been modified; and he attributes the modification to 
the change of circumstances. The author (1855) has also 
treated Psychology on the principle of the necessary acquire- 
ipent of each mental power and capacity by gradation. 

La 1852 M. Naudin, a distinguished botanist, expressly 
stated, in an admirable paper on the Origin of Species 
('Revue Horticole,' p. 102; since partly republished in the 
'Nouvelles Archives du Museum,' tom. i. p. 171), his belief 
that species are formed in an analogous manner as varieties 
are under cultivation ; and the latter process he attributes to 
man's power of selection. But he does not show how selec- 
tion acts under nature. He believes, like Dean Herbert, that 
species, when nascent, were more plastic than at present. 
He lays weight on what he calls the principle of finality; 


"puissance mysterieuse, indeterminee ; fatalite pour Ics uns ; 
pour les autres, volonte providentielle, dont Taction inces- 
sante sur les etres vivants determine, a toutes les epoques de 
I'existence du monde, la forme, le volume, et la duree de 
chacun d'eux, en raison de sa destinee dans I'orde de choses 
dont il fait partie. C'est cette puissance qui harmonise 
chaque membre a I'ensemble, en I'appropriant a la fonction 
qu'il doit remplir dans I'organisme general de la nature, fonc- 
tion qui est pour lui sa raison d'etre."* 

In 1853 a celebrated geologist, Count Keyserling ('Bulletin 
de la Soc. .Geolog./ 2nd Sen, torn. x. p. 357), suggested that 
as new diseases, supposed to have been caused by some 
miasma, have arisen and spread over the world, so at certain 
periods the germs of existing species may have been chem- 
ically afifected by circumambient molecules of a particular 
nature, and thus have given rise to new forms. 

In this same year, 1853, Dr. Schaaffhausen published an 
excellent pamphlet ('Verhand. des Naturhist. Vereins der 
Preuss. Rheinlands,' &c.), in which he maintains the devel- 
opment of organic forms on the earth. He infers that many 
species have kept true for long periods, whereas a few have 
become modified. The distinction of species he explains by 
the destruction of intermediate graduated forms. "Thus 
living plants and animals are not separated from the extinct 
by new creations, but are to be regarded as their descendants 
through continued reproduction." 

A well-known French botanist, M. Lecoq, writes in 1854 
('Etudes sur Geograph. Bot.,' torn. i. p. 250), "On voit que 
nos recherches sur la fixite ou la variation de I'espece, nous 
conduisent directement aux idees emises, par deux hommes 
justement celebres, Geoffroy Saint-Hilaire et Goethe." Some 

* From references in Bronn's ' Untersuchungen iiber die Entwickclungs- 
Gesetze,' it appears that the celebrated botanist and palxontologist Unger 
published, in 1852, his belief that species undergo development and modifi- 
cation. Dalton, likewise, in Pander and Dalton's work on Fossil Sloths, ex- 
pressed, in 1 82 1, a similar belief. Similar views have, as is well known, 
been maintained by Oken in his mystical ' Natur-Philosophie.' From other 
references in Godron's work ' Sur I'Espece,' it seems that Bory St. Vincent, 
Burdach, Poiret, and Fries, have all admitted that new species arc con- 
tinually being produced. 

I may add, that of the thirty-four authors named in this Historical Sketch, 
who believe in the modification of species, or at least disbelieve in separate 
acts of creation, twenty-seven have written on special branches of natural 
history or geology. 


other passages scattered through M. Lecoq's large work, 
make it a little doubtful how far he extends his views on the 
modification of species. 

The 'Philosophy of Creation' has been treated in a mas- 
terly manner by the Rev. Baden Powell, in his 'Essays on the 
Unity of Worlds,' 1855. Nothing can be more striking than 
the manner in which he shows that the introduction of new 
species is "a regular, not a casual phenomenon," or, as Sir 
John Herschel expresses it, "a natural in contradistinction to 
a miraculous process." 

The third volume of the 'Journal of the Linnean Society' 
contains papers, read July ist, 1858, by Mr. Wallace and my- 
self, in which, as stated in the introductory remarks to this 
volume, the theory of Natural Selection is promulgated by 
Mr. Wallace with admirable force and clearness. 

Von Baer, towards whom all zoologists feel so profound a 
respect, expressed about the year 1859 (see Prof. Rudolph 
Wagner, 'Zoologisch-Anthropologische Untersuchungen,* 
1861, s. 51) his conviction, chiefly grounded on the laws of 
geographical distribution, that forms now perfectly distinct 
have descended from a single parent-form. 

In June, 1859, Professor Huxley gave a lecture before the 
Royal Institution on the 'Persistent Types of Animal Life.' 
Referring to such cases, he remarks, "It is diflicult to com- 
prehend the meaning of such facts as these, if we suppose 
that each species of animal and plant, or each great type of 
organisation, was formed and placed upon the surface of the 
globe at long intervals by a distinct act of creative power; 
and it is well to recollect that such an assumption is as un- 
supported by tradition or revelation as it is opposed to the 
general analogy of nature. If, on the other hand, we view 
'Persistent Types' in relation to that hypothesis which sup- 
poses the species living at any time to be the result of the 
gradual modification of pre-existing species a hypothesis 
v/hicli, though unproven, and sadly damaged by some of it.s 
supporters, is yet the only one to which physiology lends any 
countenance; their existence would seem to show that the 
amount of modification which living beings have undergone 
during geological time is but very small in relation to the 
whole series of changes which they have sufifered." 


In December, 1859, Dr. Hooker published his 'Introduction 
to the Australian Flora.' In the first part of this great work 
he admits the truth of the descent and modification of spe- 
cies, and supports this doctrine by many original observa- 

The first edition of this work was published on November 
24th, 1859, and the second edition on January 7th, i860. 


When on board H.M.S. 'Beagle,' as naturalist, I was much struck 
with certain facts in the distribution of the organic beings in- 
habiting South America, and in the geological relations of the 
present to the past inhabitants of that continent. These facts, as 
will be seen in the latter chapters of this volume, seemed to throw 
some light on the origin of species — that mystery of mysteries, as 
it has been called by one of our greatest philosophers. On my 
return home, it occurred to me, in 1837, that something might 
perhaps be made out on this question by patiently accumulating 
and reflecting on all sorts of facts which could possibly have any 
bearing on it. After five years' work I allowed myself to specu- 
late on the subject, and drew up some short notes; these I 
enlarged in 1844 into a sketch of the conclusions, which then 
seemed to me probable; from that period to the present day I 
have steadily pursued the same object. I hope that I may be 
excused for entering on these personal details, as I give them to 
show that I have not been hasty in coming to a decision. 

My work is now (1859) nearly finished; but as it will take me 
many more years to complete it, and as my health is far from 
strong, I have been urged to publish this Abstract. I have more 
especially been induced to do this, as Mr. Wallace, who is now 
studying the natural history of the Malay archipelago, has arrived 
at almost exactly the same general conclusions that I have on the 
origin of species. In 1858 he sent me a memoir on this subject, 
with a request that I would forward it to Sir Charles Lyell, who 
sent it to the Linnean Society, and it is published in the third 
volume of the Journal of that Society. Sir C. Lyell and Dr. 
Hooker, who both knew of my work— the latter having read my 
sketch of 1844 — honoured me by thinking it advisable to publish, 
with Mr. Wallace's excellent memoir, some brief extracts from 
my manuscripts. 

This Abstract, which I now publish, must necessarily be im- 
perfect. I cannot here give references and authorities for my 
several statements ; and I must trust to the reader reposing some 
confidence in my accuracy. No doubt errors will have crept in. 
though I hope I have always been cautious in trusting to good 



authorities alone. I can here give only the general conclusions at 
which I have arrived, with a few facts in illustration, but which, 
I hope, in most cases will suffice. No one can feel more sensible 
than I do of the necessity of hereafter publishing in detail all the 
facts, with references, on which my conclusions have been 
grounded; and I hope in a future work to do this. For I am 
well aware that scarcely a single point is discussed in this volume 
on which facts cannot be adduced, often apparently leading to 
conclusions directly opposite to those at which I have arrived. 
A fair result can be obtained only by fully stating and balancing 
the facts and arguments on both sides of each question; and this 
is here impossible. 

I much regret that want of space prevents my having the satis- 
faction of acknowledging the generous assistance which I have 
received from very many naturalists, some of them personally 
unknown to me. I cannot, however, let this opportunity pass 
without expressing my deep obligations to Dr. Hooker, who, for 
the last fifteen years, has aided me in every possible way by his 
large stores of knowledge and his excellent judgment. 

In considering the Origin of Species, it is quite conceivable that 
a naturalist, reflecting on the mutual affinities of organic beings, 
on their embryological relations, their geographical distribution, 
geological succession, and other such facts, might come to the con- 
clusion that species have not been independently created, but had 
descended, like varieties, from other species. Nevertheless, such 
a conclusion, even if well founded, would be unsatisfactory, until 
it could be shown how the innumerable species inhabiting this 
world have been modified, so as to acquire that perfection of 
structure and coadaptation which justly excites our admiration. 
Naturalists continually refer to external conditions, such as 
climate, food, etc., as the only possible cause of variation. In one 
limited sense, as we shall hereafter see, this may be true; but 
it is preposterous to attribute to mere external conditions, the 
structure, for instance, of the woodpecker, with its feet, tail, beak, 
and tongue, so admirably adapted to catch insects under the bark 
of trees. In the case of the mistletoe, which draws its nourish- 
ment from certain trees, which has seeds that must be trans- 
ported by certain birds, and which has flowers with separate 
sexes absolutely requiring the agency of certain insects to bring 
pollen from one flower to the other, it is equally preposterous to 


account for the structure of this parasite, with its relations to 
several distinct organic beings, by the effects of external condi- 
tions, or of habit, or of the volition of the plant itself. 

It is, therefore, of the highest importance to gain a clear in- 
sight into the means of modification and coadaptation. At the 
commencement of my observations it seemed to me probable that 
a careful study of domesticated animals and of cultivated plants 
would offer the best chance of making out this obscure problem. 
Nor have I been disappointed ; in this and in all other perplexing 
cases I have invariably found that our knowledge, imperfect 
though it be, of variation under domestication, afforded the best 
and safest clue. I may venture to express my conviction of the 
high value of such studies, although they have been very com- 
monly neglected by naturalists. 

From these considerations, I shall devote the first chapter of 
this Abstract to Variation under Domestication. We shall thus 
see that a large amount of hereditary modification is at least pos- 
sible; and, what is equally or more important, we shall see how 
great is the power of man in accumulating by his Selection suc- 
cessive slight variations. I will then pass on the variability of 
species in a state of nature; but I shall, unfortunately, be 
compelled to treat this subject far too briefly, as it can be treated 
properly only by giving long catalogues of facts. We shall, how- 
ever, be enabled to discuss what circumstances are most favour- 
able to variation. In the next chapter the Struggle for Existence 
amongst all organic beings throughout the world, which inevitably 
follows from the high geometrical ratio of their increase, will be 
considered. This is the doctrine of Malthus, applied to the whole 
animal and vegetable kingdoms. As many more individuals of 
each species are born than can possibly survive; and as, conse- 
quently, there is a frequently recurrent struggle for existence, it 
follows that any being, if it vary however slightly in any manner 
profitable to itself, under the complex and sometimes varying con- 
ditions of life, will have a better chance of surviving, and thus be 
naturally selected. From the strong principle of inheritance, any 
selected variety will tend to propagate its new and modified form. 

This fundamental subject of Natural Selection will be treated 
at some length in the fourth chapter; and we shall then see how 
Natural Selection almost inevitably causes much Extinction of the 
less improved forms of life, and leads to what I have called Diver- 


gence of Character. In the next chapter I shall discuss the com- 
plex and little known laws of variation. In the five succeeding 
chapters, the most apparent and gravest difficulties in accepting 
the theory will be given : namely, first, the difficulties of transi- 
tions, or how a simple being or a simple organ can be changed 
and perfected into a highly developed being or into an elaborately 
constructed organ; secondly, the subject of Instinct, or the mental 
powers of animals ; thirdly. Hybridism, or the infertility of species 
and the fertility of varieties when intercrossed ; and fourthly, the 
imperfection of the Geological Record. In the next chapter I 
shall consider the geological succession of organic beings through- 
out time; in the twelfth and thirteenth, their geographical distri- 
bution throughout space ; in the fourteenth, their classification or 
mutual affinities, both when mature and in an embryonic condi- 
tion. In the last chapter I shall give a brief recapitulation of the 
whole work, and a few concluding remarks. 

No one ought to feel surprise at much remaining as yet unex- 
plained in regard to the origin of species and varieties, if he make 
due allowance for our profound ignorance in regard to the mutual 
relations of the many beings which live around us. Who can 
explain why one species ranges widely and is very numerous, and 
why another allied species has a narrow range and is rare? Yet 
these relations are of the highest importance, for they determine 
the present welfare, and, as I believe, the future success and 
modification of every inhabitant of this world. Still less do we 
know of the mutual relations of the innumerable inhabitants of 
the world during the many past geological epochs in its history. 
Although much remains obscure, and will long remain obscure, I 
can entertain no doubt, after the most deliberate study and dis- 
passionate judgment of which I am capable, that the view which 
most naturalists until recently entertained, and which I formerly 
entertained — namely, that each species has been independently 
created — is erroneous. I am fully convinced that species are not 
immutable ; but that those belonging to what are called the same 
genera are lineal descendants of some other and generally extinct 
species, in the same manner as the acknowledged varieties of any 
one species are the descendants of that species. Furthermore, I 
am convinced that Natural Selection has been the most important, 
but not the exclusive, means of modification. 


Variation under Domestication 

Causes of variability — Effects of habit and the use or disuse of parts- 
Correlated variation — Inheritance — Character of domestic varie- 
ties — Difificulty of distinguishing between varieties and species — 
Origin of domestic varieties from one or more species — Domestic 
pigeons, their differences and origin — Principles of selection, an- 
ciently followed, their efifects — Methodical and unconscious 
selection — Unknown origin of our domestic productions — Circum- 
stances favourable to man's power of selection 


WHEN we compare the individuals of the same 
variety or sub-variety of our older cultivated plants 
and animals, one of the first points which strikes 
us is, that they generally differ more from each other than 
do the individuals of any one species or variety in a state of 
nature. And if we reflect on the vast diversity of the plants 
and animals which have been cultivated, and which have 
varied during all ages under the most different climates and 
treatment, we are driven to conclude that this great varia- 
bility is due to our domestic productions having been raised 
under conditions of life not so uniform as, and somewhat 
different from, those to which the parent species had been 
exposed under nature. There is, also, some probability in 
the view propounded by Andrew Knight, that this variability 
may be partly connected with excess of food. It seems clear 
that organic beings must be exposed during several genera- 
tions to new conditions to cause any great amount of varia- 
tion; and that, when the organisation has once begun to 
vary, it generally continues varying for many generations. 
No case is on record of a variable organism ceasing to vary 
wnder cultivation. Our oldest cultivated plants, such as 



wheat, still yield new varieties : our oldest domesticated ani- 
mals are still capable of rapid improvement or modification. 

As far as I am able to judge, after long attending to the 
subject, the conditions of life appear to act in two ways, — • 
directly on the whole organisation or on certain parts alone, 
and indirectly by affecting the reproductive system. With re- 
spect to the direct action, we must bear in mind that in every 
case, as Professor Weismann has lately insisted, and as I have 
incidentally shown in my work on 'Variation under Domesti- 
cation,' there are two factors; namely, the nature of the 
organism, and the nature of the conditions. The former 
seems to be much the more important; for nearly similar 
variations sometimes arise under, as far as we can judge, 
dissimilar conditions; and, on the other hand, dissimilar 
variations arise under conditions which appear to be nearly 
uniform. The effects on the offspring are either definite or 
indefinite. They may be considered as definite when all or 
nearly all the offspring of individuals exposed to certain 
conditions during several generations are modified in the 
same manner. It is extremely difificult to come to any con- 
clusion in regard to the extent of the changes which have 
been thus definitely induced. There can, however, be little 
doubt about many slight changes, — such as size from the 
amount of food, colour from the nature of the food, thick- 
ness of the skin and hair from climate, etc. Each of the 
endless variations which we see in the plumage of our fowls 
must have had some efficient cause; and if the same cause 
were to act uniforml}'^ during a long series of generations on 
many individuals, all probably would be modified in the 
same manner. Such facts as the complex and extraordinary 
out-growths which variably follow from the insertion of a 
minute drop of poison by a gall-producing insect, show us 
what singular modifications might result in the case of plants 
from a chemical change in the nature of the sap. 

Indefinite variability is a much more common result of 
changed conditions than definite variability, and has prob- 
ably played a more important part in the formation of our 
domestic races. We see indefinite variability in the endless 
slight peculiarities which distinguish the individuals of the 
same species, and which cannot be accounted for by inher- 


ilance from either parent or from some more remote ances- 
tor. Even strongly-marked differences occasionally appear 
in the young of the same litter, and in seedlings from the 
same seed capsule. At long intervals of time, out of millions 
of individuals reared in the same country and fed on nearly 
the same food, deviations of structure so strongly pro- 
nounced as to deserve to be called monstrosities arise; but 
monstrosities cannot be separated by any distinct line from 
slighter variations. All such changes of structure, whether 
extremely slight or strongly marked, which appear amongst 
many individuals living together, may be considered as the 
indefinite effects of the conditions of life on each individual 
organism, in nearly the same manner as the chill affects dif- 
ferent men in an indefinite manner, according to their state 
of body or constitution, causing coughs or colds, rheumatism, 
or inflammation of various organs. 

With respect to what I have called the indirect action of 
changed conditions, namely, through the reproductive sys- 
tem of being affected, we may infer that variability is thus 
induced, partly from the fact of this system being extremely 
sensitive to any change in the conditions, and partly 
from the similarity, as Kolreuter and others have re- 
marked, between the variability which follows from the 
crossing of distinct species, and that which may be ob- 
served with plants and animals when reared under new 
or unnatural conditions. Many facts clearly show how 
eminently susceptible the reproductive system is to very 
slight changes in the surrounding conditions. Nothing is 
more easy than to tame an animal, and few things more diffi- 
cult than to get it to breed freely under confinement, even 
when the male and female unite. How many animals there 
are which will not breed, though kept in an almost free state 
in their native country ! This is generally, but erroneously, 
attributed to vitiated instincts. Many cultivated plants dis- 
play the utmost vigour, and yet rarely or never seed ! In 
some few cases it has been discovered that a very trifling 
change, such as a little more or less water at some particular 
period of growth, will determine whether or not a plant will 
produce seeds. I cannot here give the details which I have 
collected and elsewhere published on this curious subject; 


but to show how singular the laws are which determine thei 
reproduction of animals under confinement, I may mention 
that carnivorous animals, even from the tropics, breed in 
this country pretty freely under confinement, with the excep- 
tion of the plantigrades or bear family, which seldom pro- 
duce young; whereas carnivorous birds, with the rarest ex- 
ceptions, hardly ever lay fertile eggs. Many exotic plants 
have pollen utterly worthless, in the same condition as in the 
most sterile hybrids. When, on the one hand, we see domes- 
ticated animals and plants, though often weak and sickly, 
breeding freely tmder confinement; and when, on the other 
hand, we see individuals, though taken young from a state of 
nature perfectly tamed, long-lived and healthy (of which I 
could give numerous instances), yet, having their repro- 
ductive system so seriously affected by unperceived causes as 
to fail to act, we need not be surprised at this system, when 
it does act under confinement, acting irregularly, and pro- 
ducing offspring somewhat unlike their parents. I may add, 
that as some organisms breed freely under the most unnat- 
ural conditions (for instance, rabbits and ferrets kept in 
hutches), showing that their reproductive organs are not 
easily affected ; so will some animals and plants withstand 
domestication or cultivation, and vary very slightly — per- 
haps hardly more than in a state of nature. 

Some naturalists have maintained that all variations are 
connected with the act of sexual reproduction; but this is 
certainly an error; for I have given in another work a long 
list of "sporting plants," as they are called by gardeners; — 
that is, of plants which have suddenly produced a single bud 
with a new and sometimes widely different character from 
that of the other buds on the same plant. These bud-varia- 
tions, as they may be named, can be propagated by grafts, 
offsets, etc., and sometimes by seed. They occur rarely 
under nature, but are far from rare under culture. As a 
single bud out of the many thousands, produced year after 
year on the same tree under uniform conditions, has been 
known suddenly to assume a new character ; and as buds on 
distinct trees, growing under different conditions, have some- 
times yielded nearly the same variety — for instance, buds on 
peach-trees producing nectarines, and buds on common roses 


producing moss-roses — we clearly see that the nature of the 
condition is of subordinate importance in comparison with 
the nature of the organism in determining each particular 
form of variation — perhaps of not more importance than the 
nature of the spark, by which a mass of combustible matter 
is ignited, has in determining the nature of the flames. 


Changed habits produce an inherited efifect, as in the pe- 
riod of the flowering of plants when transported from one 
climate to another. With animals the increased use or dis- 
use of parts has had a more marked influence; thus I find in 
the domestic duck that the bones of the wing weigh less and 
the bones of the leg more, in proportion to the whole skele- 
ton, than do the same bones in the wild duck; and this 
change may be safely attributed to the domestic duck flying 
much less, and walking more, than its wild parents. The 
great and inherited development of the udders in cows and 
goats in countries where they are habitually milked, in com- 
parison with these organs in other countries, is probably 
another instance of the effects of use. Not one of our do- 
mestic animals can be named which has not in some country 
drooping ears ; and the view which has been suggested that 
the drooping is due to disuse of the muscles of the ear, from 
the animals being seldom much alarmed, seems probable. 

Many laws regulate variation, some few of which can be 
dimly seen, and will hereafter be briefly discussed. I will 
here only allude to what may be called correlated variation. 
Important changes in the embryo or larva will probably en- 
tail changes in the mature animal. In monstrosities, the 
correlations between quite distinct parts are very curious; 
and many instances are given in Isidore Geoff roy St. 
Hilaire's great work on this subject. Breeders believe that 
long limbs are almost always accompanied by an elongated 
head. Some instances of correlation are quite whimsical : 
thus cats which are entirely white and have blue eyes are 
generally deaf ; but it has been lately stated by Mr. Tait that 
this is confined to the males. Colour and constitutional pecu- 


liarities go together, of which many remarkable cases could 
be given amongst animals and plants. From facts collected 
by Heusinger, it appears that white sheep and pigs are in- 
jured by certain plants, whilst dark-coloured individuals es- 
cape : Professor Wyman has recently communicated to me 
a good illustration of this fact; on asking some farmers in 
Virginia how it was that all their pigs were black, they in- 
formed him that the pigs ate the paint-root (Lachnanthes), 
which colored their bones pink, and which caused the hoofs 
of all but the black varieties to drop off; and one of the 
"crackers" (i.e. Virginia squatters) added, "we select the 
black members of a litter for raising, as they alone have a 
good chance of living.'' Hairless dogs have imperfect teeth ; 
long-haired and coarse-haired animals are apt to have, as is 
asserted, long or many horns ; pigeons with feathered feet 
have skin between their outer toes ; pigeons with short beaks 
have small feet, and those with long beaks large feet. Hence 
if man goes on selecting, and thus augmenting, any pecu- 
liarity, he will almost certainly modify unintentionally other 
parts of the structure, owing to the mysterious laws of cor- 

The results of the various, unknown, or but dimly under- 
stood laws of variation are infinitely complex and diversified. 
It is well worth whde carefully to study the several treatises 
on some of our old cultivated plants, as on the hyacinth, 
potato, even the dahlia, etc. ; and it is really surprising to 
note the endless points of structure and constitution in which 
the varieties and sub-varieties differ slightly from each 
other. The whole organisation seems to have become 
plastic, and departs in a slight degree from that of the 
parental type. 

Any variation which is not inherited is unimportant for 
us. But the number and diversity of inheritable deviations 
of structure, both those of slight and those of considerable 
physiological importance, are endless. Dr. Prosper Lucas's 
treatise, in two large volumes, is the fullest and the best on 
this subject. No breeder doubts how strong is the tendency 
to inheritance; that like produces like is his fundamental be- 
lief: doubts have been thrown on this principle only by theo- 
retical writers. When any deviation of structure often 



appears, and we see it in the father and child, we cannot tell 
whether it may not be due to the same cause having acted on 
both ; but when amongst individuals, apparently exposed to 
the same conditions, any very rare deviation, due to some 
extraordinary combination of circumstances, appears in the 
parent — say, once amongst several million individuals — and 
it reappears in the child, the mere doctrine of chances almost 
compels us to attribute its reappearance to inheritance. 
Every one must have heard of cases of albinism, prickly 
skin, hairy bodies, etc., appearing in several members of the 
same family. If strange and rare deviations of structure are 
really inherited, less strange and commoner deviations may 
be freely admitted to be inheritable. Perhaps the correct 
way of viewing the whole subject would be, to look at the 
inheritance of every character whatever as the rule, and 
non-inheritance as the anomaly. 

The laws governing inheritance are for the most part 
unknown. No one can say why the same peculiarity in dif- 
ferent individuals of the same species, or in different species, 
is sometimes inherited and sometimes not so; why the child 
often reverts in certain characters to its grandfather or 
grandmother or more remote ancestor; why a peculiarity is 
often transmitted from one sex to both sexes, or to one sex 
alone, more commonly but not exclusively to the like sex. 
It is a fact of some importance to us, that peculiarities ap- 
pearing in the males of our domestic breeds are often trans- 
mitted, either exclusively or in a much greater degree, to 
the males alone. A much more important rule, which I 
think may be trusted, is that, at whatever period of life a 
peculiarity first appears, it tends to reappear in the offspring 
at a corresponding age, though sometimes earlier. In many 
cases this could not be otherwise ; thus the inherited pecu- 
liarities in the horns of cattle could appear only in the off- 
spring when nearly mature ; peculiarities in the silkworm 
are known to appear at the corresponding caterpillar or 
cocoon stage. But hereditary diseases and some other facts 
make me believe that the rule has a wider extension, and 
that, when there is no apparent reason why a peculiarity 
should appear at any particular age, yet that it does tend to 
appear in the offspring at the same period at which it first 


appeared in the parent. I believe this rule to be of the 
highest importance in explaining the laws of embryology. 
These remarks are of course confined to the first appearance 
of the peculiarity, and not to the primary cause which may 
have acted on the ovules or on the male element; in nearly 
the same manner as the increased length of the horns in 
the offspring from a short-horned cow by a long-horned 
bull, though appearing late in life, is clearly due to the male 

Having alluded to the subject of aversion, I may here 
refer to a statement often made by naturalists — namely, 
that our domestic varieties, when run wild, gradually but 
invariably revert in character to their aboriginal stocks. 
Hence it has been argued that no deductions can be drawn 
from domestic races to species in a state of nature. I have 
in vain endeavoured to discover on what decisive facts the 
above statement has so often and so boldly been made. 
There would be great difficulty in proving its truth : we may 
safely conclude that very many of the most strongly marked 
domestic varieties could not possibly live in a wild state. 
In many cases we do not know what the aboriginal stock 
M-as, and so could not tell whether or not nearly perfect re- 
version had ensued. It would be necessary, in order to pre- 
vent the effects of intercrossing, that only a single variety 
should have been turned loose in its new home. Neverthe- 
less, as our varieties certainly do occasionally revert in some 
of their characters to ancestral forms, it seems to me not 
improbable that if we could succeed in naturalising, or were 
to cultivate, during many generations, the several races, for 
instance, of the cabbage, in very poor soil (in which case, 
however, some effect would have to be attributed to the 
definite action of the poor soil), that they would, to a large 
extent, or even wholly, revert to the wild aboriginal stock. 
Whether or not the experiment would succeed, is not of 
great importance for our line of argument; for by the ex- 
periment itself the conditions of life are changed. If it 
could be shown that our domestic varieties manifested a 
strong tendency to reversion, — that is, to lose their acquired 
characters, whilst kept under the same conditions, and whilst 
kept in a considerable body, so that free intercrossing might 


check, by blending together, any slight deviations in their 
structure, in such case, I grant that we could deduce nothing 
from domestic varieties in regard to species. But there is 
not a shadow of evidence in favour of this view : to assert 
that we could not breed our cart- and race-horses, long- and 
short-horned cattle, and poultry of various breeds, and escu- 
lent vegetables, for an unlimited number of generations, 
would be opposed to all experience. 






When we look to the hereditary varieties or races of our 
domestic animals and plants, and compare them with closely 
allied species, we generally perceive in each domestic race, 
as already remarked, less uniformity of character than in 
true species. Domestic races often have a somewhat mon- 
strous character; by which I mean, that, although differing 
from each other, and from other species of the same genus, 
in several trifling respects, they often differ in an extreme 
degree in some one part, both when compared one with an- 
other, and more especially when compared with the species 
under nature to which they are nearest allied. With these 
exceptions (and with that of the perfect fertility of varieties 
when crossed, — a subject hereafter to be discussed), domes- 
tic races of the same species differ from each other in the 
same manner as do the closely allied species of the same 
genus in a state of nature, but the differences in most cases 
are less in degree. This must be admitted as true, for the 
domestic races of many animals and plants have been ranked 
by some competent judges as the descendants of aboriginally 
distinct species, and by other competent judges as mere 
varieties. If any well-marked distinction existed between a 
domestic race and a species, this source of doubt would not 
so perpetually recur. It has often been stated that domestic 
races do not differ from each other in characters of generic 
value. It can be shown that this statement is not correct; 



but naturalists differ much in determining what characters 
are of generic value; all such valuations being at present 
empirical. When it is explained how genera originate under 
nature, it will be seen that we have no right to expect often 
to find a generic amount of difference in our domesticated 

In attempting to estimate the amount of structural differ- 
ence between allied domestic races, we are soon involved 
in doubt, from not knowing whether they are descended from 
one or several parent species. This point, if it could be 
cleared up, would be interesting; if, for instance, it could be 
shown that the greyhound, bloodhound, terrier, spaniel, and 
bulldog, which we all know propagate their kind truly, were 
the offspring of any single species, then such facts would 
have great weight in making us doubt about the immuta- 
bility of the many closely allied natural species — for in- 
stance, of the many foxes — inhabiting different quarters of 
the world. I do not believe, as we shall presently see, that 
the whole amount of difference between the several breeds 
of the dog has been produced under domestication ; I believe 
that a small part of the difference is due to their being 
descended from distinct species. In the case of strongly 
marked races of some other domesticated species, there 
is presumptive or even strong evidence, that all are 
descended from a single wild stock. 

It has often been assumed that man has chosen for domes- 
tication animals and plants having an extraordinary inherent 
tendency to vary, and likewise to withstand diverse climates. 
I do not dispute that these capacities have added largely to 
the value of most of our domesticated productions; but how 
could a savage possibly know, when he first tamed an ani- 
mal, whether it would vary in succeeding generations, and 
whether it would endure other climates? Has the little 
variability of the ass and goose, or the small power of en- 
durance of warmth by the reindeer, or of cold by the com- 
mon camel, prevented their domestication? I cannot doubt 
that if other animals and plants, equal in number to our 
domesticated productions, and belonging to equally diverse 
classes and countries, were taken from a state of nature, 
and could be made to breed for an equal number of genera- 


tions under domestication, they would on an average vary as 
largely as the parent species of our existing domesticated 
productions have varied. 

In the case of most of our anciently domesticated animals 
and plants, it is not possible to come to any definite con- 
clusion, whether they are descended from one or several 
wild species. The argument mainly relied on by those who 
believe in the multiple origin of our domestic animals is, 
that we find in the most ancient times, on the monuments 
of Egypt, and in the lake-habitations of Switzerland, much 
diversity in the breeds; and that some of these ancient breeds 
closely resemble, or are even identical with, those still ex- 
isting. But this only throws far backwards the history of 
civilisation, and shows that animals were domesticated at a 
much earlier period than has hitherto been supposed. The 
lake-inhabitants of Switzerland cultivated several kinds of 
wheat and barley, the pea, the poppy for oil, and flax; and 
they possessed several domesticated animals. They also 
carried on commerce with other nations. All this clearly 
shows, as Heer has remarked, that they had at this early 
age progressed considerably in civilisation ; and this again 
implies a long continued previous period of less advanced 
civilisation, during which the domesticated animals, kept 
by different tribes in different districts, might have varied 
and given rise to distinct races. Since the discovery of 
flint tools in the superficial formations of many parts of 
the world, all geologists believe that barbarian man existed 
at an enormously remote period and we know that at the 
present day there is hardly a tribe so barbarous, as not to 
have domesticated at least the dog. 

The origin of most of our domestic animals will prob- 
ably for ever remain vague. But I may here state, that, 
looking to the domestic dogs of the whole world. I have, 
after a laborious collection of all known facts, come to the 
conclusion that several wild species of Canidc'c have been 
tamed, and that their blood, in some cases mingled together, 
flows in the veins of our domestic breeds. In regard to 
sheep and goats I can form no decided opinion. From facts 
communicated to me by Mr. Blyth, on the habits, voice, con- 
stitution, and structure of the humped Indian cattle, it is 


almost certain that they are descended from a different abo- 
rigmal stock from our European cattle and some competent 
judges believe that these latter have had two or three wild 
progenitors, — whether or not these deserve to be called 
species. This conclusion, as well as that of the specific dis- 
tinction between the humped and common cattle, may, in- 
deed, be looked upon as established by the admirable re- 
searches of Professor Riitimeyer. With respect to horses, 
from reasons which I cannot here give, I am doubtfully 
inclined to believe, in opposition to several authors, that all 
the races belong to the same species. Having kept nearly 
all the English breeds of the fowl alive, having bred and 
crossed them, and examined their skeletons, it appears to 
me almost certain that all are the descendants of the wild 
Indian fowl, Gallus bankiva; and this is the conclusion of 
Mr. Blyth, and of others who have studied this bird in 
India. In regard to ducks, and rabbits, some breeds of which 
differ much from each other, the evidence is clear that they 
are all descended from the common wild duck and rabbit. 

The doctrine of the origin of our several domestic races 
from several aboriginal stocks, has been carried to an absurd 
extreme by some authors. They believe that every race 
which breeds true, let the distinctive characters be ever so 
slight, has had its wild prototype. At this rate there must 
have existed at least a score of species of wild cattle, as 
many sheep, and several goats, in Europe alone, and several 
even within Great Britain. One author believes that there 
formerly existed eleven wild species of sheep peculiar to 
Great Britain ! When we bear in mind that Britain has now 
not one peculiar mammal, and France but few distinct from 
those of Germany, and so with Hungary. Spain, etc., but 
that each of these kingdoms possesses several peculiar breeds 
of cattle, sheep, etc., we must admit that many domestic 
breeds must have originated in Europe; from whence other- 
wise could they have been derived? So it is in India. Even 
in the case of the breeds of the domestic dog throughout the 
world, which I admit are descended from several wild spe- 
cies, it cannot be doubted that there has been an immense 
amount of inherited variation ; for who will believe that 
animals closely resembling the Italian greyhound, the blood- 


hound, the bull-dog, pug-dog, or Blenheim spaniel, etc. — so 
unlike all wild Canidae — ever existed in a state of nature? 
It has often been loosely said that all our races of dogs 
have been produced by the crossing of a few aboriginal spe- 
cies ; but by crossing we can only get forms in some degree 
intermediate between their parents ; and if we account for 
our several domestic races by this process, we must admit 
the former existence of the most extreme forms, as the 
Italian greyhound, bloodhound, bull-dog, etc., in the wild 
state, IMoreover, the possibility of making distinct races by 
crossing has been greatly exaggerated. ^lany cases are on 
record, showing that a race may be modified by occasional 
crosses, if aided by the careful selection of the individuals 
which present the desired character ; but to obtain a race 
intermediate between two quite distinct races, would be very 
difficult. Sir J, Sebright expressly experimented with this 
object and failed. The offspring from the first cross be- 
tween two pure breeds is tolerably and sometimes (as I have 
found with pigeons) quite uniform in character, and every- 
thing seems simple enough ; but when these mongrels are 
crossed one with another for several generations, hardly 
two of them are alike, and then the difficulty of the task 
becomes manifest. 



Believing that it is always best to study some special 
group, I have, after deliberation, taken up domestic pigeons. 
I have kept every breed which I could purchase or obtain, 
and have been most kindly favoured with skins from several 
quarters of the world, more especially by the Hon. W. Elliot 
from India, and by the Hon. C. Murray from Persia. Many 
treatises in different languages have been published on pig- 
eons, and some of them are very important, as being of con- 
siderable antiquity. I have associated with several eminent 
fanciers, and have been permitted to join two of the London 
Pigeon Clubs. The diversity of the breeds is something as- 
tonishing. Compare the English carrier and the short-faced 
tumbler, and see the wonderful difference in their beaks. 


entailing corresponding differences in their skulls. The 
carrier, more especially the male bird, is also remarkable 
from the wonderful development of the carunculated skin 
about the head ; and this is accompanied by greatly elongated 
eyelids, very large external orifices to the nostrils, and a 
vv^ide gape of mouth. The short- faced tumbler has a beak 
in outline almost like that of a finch; and the common 
tumbler has the singular inherited habit of flying at a great 
height in a compact flock, and tumbling in the air head over 
heels. The runt is a bird of great size, with long massive 
beak and large feet; some of the sub-breeds of runts have 
very long necks, others very long wings and tails, others 
singularly short tails. The barb is allied to the carrier, but, 
instead of a long beak, has a very short and broad one. The 
pouter has a much elongated body, wings, and legs ; and 
its enormously developed crop, which it glories in inflating, 
may well excite astonishment and even laughter. The turbit 
has a short and conical beak, with a line of reversed feathers 
down the breast; and it has the habit of continually expand- 
ing, slightly, the upper part of the oesophagus. The Jacobin 
has the feathers so much rcA^ersed along the back of the neck 
that they form a hood; and it has, proportionally to its 
size, elongated wing and tail feathers. The trumpeter and 
laugher, as their names express, utter a very different coo 
from the other breeds. The fantail has thirty or even forty 
tail-feathers, instead of twelve or fourteen — the normal 
number in all the members of the great pigeon family : these 
feathers are kept expanded, and are carried so erect, that in 
good birds the head and tail touch: the oil-gland is quite 
aborted. Several other less distinct breeds might be 

In the skeletons of the several breeds, the development of 
the bones of the face in length and breadth and curvature 
differs enormously. The shape, as well as the breadth and 
length of the ramus of the lower jaw, varies in a highly 
remarkable manner. The caudal and sacral vertebrae vary 
in number; as does the number of the ribs, together with 
their relative breadth and the presence of processes. The size 
and shape of the apertures in the sternum are highly vari- 
able; so is the degree of divergence and relative size of tht 


two arms of the furcula. The porportional width of the 
gape of mouth, the proportional length of the eyelids, of the 
orifice of the nostrils, of the tongue (not always in strict 
correlation with the length of beak), the size of the crop 
and of the upper part of the oesophagus; the development 
and abortion of the oil-gland; the number of the primary 
wing and caudal feathers; the relative length of the wing 
and tail to each other and to the body; the relative length 
of the leg and foot; the number of scutellae on the toes, the 
development of skin between the toes, are all points of struct- 
ure which are variable. The period at which the perfect 
plumage is acquired varies, as does the state of the down 
with which the nestling birds are clothed when hatched. 
The shape and size of the eggs vary. The manner of flight, 
and in some breeds the voice and disposition, differs re- 
markably. Lastly, in certain breeds, the males and females 
have come to differ in a slight degree in each other. 

Altogether at least a score of pigeons might be chosen, 
which, if shown to an ornithologist, and he were told that 
they were wild birds, would certainly be ranked by him 
as well-defined species. Moreover, I do not believe that any 
ornithologist would in this case place the English carrier, 
the short-faced tumbler, the runt, the barb, pouter, and 
fantail in the same genus; more especially as in each of 
these breeds several truly inherited sub-breeds, or species, as 
he would call them, could be shown him. 

Great as are the differences between the breeds of the 
pigeon, I am fully convinced that the common opinion of 
naturalists is correct, namely, that all are descended from 
the rock-pigeon (Columba livia), including under this term 
several geographical races or sub-species, which differ from 
each other in the most trifling respects. As several of the 
reasons which have led me to this belief are in some de- 
gree applical)le in other cases, I will here briefly give them. 
If the several breeds are not varieties, and have not pro- 
ceeded from the rock-pigeon, they must have descended from 
at least seven or eight aboriginal stocks ; for it is impossible 
to make the present domestic breeds by the crossing of any 
lesser number: how, for instance, could a pouter be produced 
by crossing two breeds unless one of the parent-stocks pes- 


sessed the characteristic enormous crop ? The supposed abo- 
riginal stocks must all have been rock-pigeons, that is, they 
did not breed or willingly perch on trees. But besides C. 
livia, with its geographical sub-species, only two or three 
Other species of rock-pigeons are known and these have 
not any of the characters of the domestic breeds. Hence 
the supposed aboriginal stocks must either still exist in the 
countries where they were originally domesticated, and yet 
be unknown to ornithologists; and this, considering their 
size, habits, and remarkable characters, seems improbable; 
or they must have become extinct in the wild state. But 
birds breeding on precipices, and good fliers, are unlikely 
to be exterminated ; and the common rock-pigeon, which has 
the same habits with the domestic breeds, has not been ex- 
terminated even on several of the smaller British islets, or 
on the shores of the Mediterranean. Hence the supposed 
extermination of so many species having similar habits with 
the rock-pigeon seems a very rash assumption. Moreover, 
the several above-named domesticated breeds have been 
transported to all parts of the world, and, therefore, some 
of them must have been carried back again into their native 
country; but not one has become wild or feral, though the 
dovecot-pigeon, which is the rock-pigeon in a very slightly 
altered state, has become feral in several places. Again, 
all recent experience shows that it is difficult to get wild ani- 
mals to breed freely under domestication; yet on the hy- 
pothesis of the multiple origin of our pigeons, it must be 
assumed that at least seven or eight species were so thor- 
oughly domesticated in ancient times by half-civilised man, 
as to be quite prolific under confinement. 

An argument of great weight, and applicable in several 
other cases, is, that the above-specified breeds, though agree- 
ing generally with the wild rock-pigeon in constitution, habits, 
voice, colouring, and in most parts of their structure, yet are 
certainly highly abnormal in other parts ; we may look in vain 
through the whole great family of Columbidje for a beak like 
that of the English carrier, or that of the short-faced tum- 
bler, or barb ; for reversed feathers like those of the Jacobin ; 
for a crop like that of the pouter; for tail-feathers like those 
of the fantail. Hence it must be assumed not only that half- 


civilised man succeeded in thorouj^hly domesticating several 
species, but that he intentionally or by chance picked out 
extraordinarily abnormal species ; and further, that these very 
species have since all become extinct or unknown. So many 
strange contingencies are improbable in the highest degree. 
Some facts in regard to the colouring of pigeons well de- 
serve consideration. The rock-pigeon is of a slaty-blue, with 
white loins;' but the Indian sub-species, C. intermedia of 
Strickland, has this part bluish. The tail has a terminal dark 
bar, with the outer feathers externally edged at the base with 
white. The wings have two black bars. Some semi-domes- 
tic breeds, and some truly wild breeds, have, besides the two 
black bars, the wings chequered with black. These several 
marks do not occur together in any other species of the whole 
family. Now, in every one of the domestic breeds, taking 
thoroughly well-bred birds, all the above marks, even to the 
white edging of the outer tail-feathers, sometimes concur 
perfectly developed. Moreover, w'hen birds belonging to two 
or more distinct breeds are crossed, none of which are blue 
or have any of the above-specified marks, the mongrel off- 
spring are very apt suddenly to acquire these characters. To — 
give one instance out of several which I have observed : — I 1 
crossed some white fantails, which breed very true, with some \ 
black barbs — and it so happens that blue varieties of barbs 
are so rare that I never heard of an instance in England; and 
the mongrels were black, brown, and mottled. I also crossed 
a barb with a spot, which is a white bird with a red tail and 
red spot on the forehead, and which notoriously breeds very 
true ; the mongrels were dusky and mottled. I then crossed 
one of the mongrel barb-fantails with a mongrel barb-spot, 
and they produced a bird of as beautiful a blue colour, with 
the white loins, double black wing-bar, and barred and white- / 
edged tail-feathers, as any wild rock-pigeon ! We can under- y* 
stand these facts, on the well-known principle of reversion to 
ancestral characters, if all the domestic breeds are descended 
from the rock-pigenn. But if we deny this, we must make 
one of the two following highly improbable suppositions. 
Either, first, that all the several imagined aboriginal stocks 
were coloured and marked like the rock-pigeon, although no 
other existing species is thus coloured and marked, so that in 


each separate breed there might be a tendency to revert to 
the very same colours and markings. Or, secondly, that each 
breed, even the purest, has within a dozen, or at most within 
a score, of generations, been crossed by the rock -pigeon; I 
say within a dozen or twenty generations, for no instance is 
known of crossed descendants reverting to an ancestor of 
foreign blood, removed by a greater number of generations. 
In a breed which has been crossed only once, the tendency to 
revert to any character derived from such a cross will nat- 
urally become less and less, as in each succeeding generation 
there will be less of the foreign blood; but when there has 
been no cross, and there is a tendency in the breed to revert 
to a character which was lost during some former genera- 
tion, this tendency, for all that we can see to the contrary, 
may be transmitted undiminished for an indefinite number of 
generations. These two distinct cases of reversion are often 
confounded together by those who have written on inheri- 

Lastly, the hybrids or mongrels from between all the breeds 
of the pigeon are perfectly fertile, as I can state from my 
own observations, purposely made, on the most distinct breeds. 
Now, hardly any cases have been ascertained with certainty 
of hybrids from two quite distinct species of animals being 
perfectly fertile. Some authors believe that long-continued 
domestication eliminates this strong tendency to sterility in 
species. From the history of the dog, and of some other do- 
mestic animals, this conclusion is probably quite correct, if 
applied to species closely related to each other. But to ex- 
tend it so far as to suppose that species, aboriginally as dis- 
tinct as carriers, tumblers, pouters, and fantails now are, 
should yield offspring perfectly fertile inter se, would be 
rash in the extreme. 

From these several reasons, namely, — the improbability of 
man having formerly made seven or eight supposed species 
of pigeons to breed freely under domestication; — these sup- 
posed species being quite unknown in a wild state, and their 
not having become anywhere feral; — these species presenting 
certain very abnormal characters, as compared with all other 
Columbidse, though so like the rock-pigeon in most respects ; 
— the occasional re-appearance of the blue colour and various 


black marks in all the breeds, both when kept pure and when 
crossed; — and lastly, the mongrel offspring being perfectly 
fertile ; — from these several reasons, taken together, we may 
safely conclude that all our domestic breeds are descended 
from the rock -pigeon or Columba livia with its geographical 

In favour of this view, I may add, firstly, that the wild C. 
livia has been found capable of domestication in Europe and 
in India ; and that it agrees in habits and in a great number 
of points of structure with all the domestic breeds. Sec- 
ondly, that, although an English carrier or a short-faced 
tumbler differs immensely in certain characters from the 
rock-pigeon, yet that, by comparing the several sub-breeds of 
these two races, more especially those brought from distant 
countries, we can make, between them and the rock-pigeon, 
an almost perfect series; so we can in some other cases, but 
not wath all the breeds. Thirdly, those characters which are 
mainly distinctive of each breed are in each eminently vari- 
able, for instance the wattle and length of beak of the carrier, 
the shortness of that of the tumbler, and the number of tail- 
feathers in the fantail ; and the explanation of this fact will 
be obvious when we treat of Selection. Fourthly, pigeons 
have been watched and tended with the utmost care, and 
loved by many people. They have been domesticated for 
thousands of years in several quarters of the world ; the ear- 
liest known record of pigeons is in the fifth Egyptian dy- 
nasty, about 3000 B.C., as was pointed out to me by Professor 
Lepsius ; but Mr. Birch informs me that pigeons are given 
in a bill of fare in the previous dynasty. In the time of the 
Romans, as we hear from Pliny, immense prices were given 
for pigeons; "nay, they are come to this pass, that they can 
reckon up their pedigree and race." Pigeons were much 
valued by Akber Khan in India, about the year 1600; never 
less than 20,000 pigeons were taken with the court. "The 
monarchs of Iran and Turan sent him some very rare birds;" 
and, continues the courtly historian, "His Majesty by cross- 
ing the breeds, which method was never practised before, has 
improved them astonishingly." About this same period the 
Dutch were as eager about pigeons as were the old Romans. 
The paramount importance of these considerations in ex- 


plaining the immense amount of variation which pigeons have 
undergone, will likewise be obvious when we treat of Selec- 
tion. We shall then, also, see how it is that the several 
breeds so often have a somewhat monstrous character. It is 
also a most favourable circumstance for the production of 
distinct breeds, that male and female pigeons can be easily 
mated for life ; and thus different breeds can be kept together 
in the same aviary. 

I have discussed the probable origin of domestic pigeons at 
some, yet quite insufficient, length ; because when I first kept 
pigeons and watched the several kinds, well knowing how 
truly they breed, I felt fully as much difficulty in believing 
that since they had been domesticated they had all proceeded 
from a common parent, as any naturalist could in coming to 
a similar conclusion in regard to the many species of finches, 
or other groups of birds, in nature. One circumstance has 
struck me much ; namely, that nearly all the breeders of the 
various domestic animals and the cultivators of plants, with 
whom I have conversed, or whose treatises I have read, are 
firmly convinced that the several breeds to which each has 
attended, are descended from so many aboriginally distinct 
species. Ask, as I have asked, a celebrated raiser of Here- 
ford cattle, whether his cattle might not have descended from 
Longhorns, or both from a common parent-stock, and he will 
laugh you to scorn. I have never met a pigeon, or poultry, 
or duck, or rabbit fancier, who was not fully convinced that 
each main breed was descended from a distinct species. Van 
Mons, in his treatise on pears and apples, shows how utterly 
he disbelieves that the several sorts, for instance a Ribston- 
pippin or Codlin-apple, could ever have proceeded from the 
seeds of the same tree. Innumerable other examples could 
be given. The explanation, I think, is simple : from long- 
continued study they are strongly impressed with the differ- 
ences between the several races ; and though they well know 
that each race varies slightly, for they win their prizes by 
selecting such slight differences, yet they ignore all general 
arguments, and refuse to sum up in their minds slight differ- 
ences accumulated during many successive generations. May 
not those naturalists who, knowing far less of the laws of 
inheritance than does the breeder, and knowing no more than 


he does of the intermediate links in the long lines of descent, 
yet admit that many of our domestic races are descended 
from the same parents — may they not learn a lesson of cau- 
tion, when they deride the idea of species in a state of nature 
being lineal descendants of other species? 



Let us now briefly consider the steps by which domestic races 
have been produced, either from one or from several allied 
species. Some effect may be attributed to the direct and defi- 
nite action of the external conditions of life, and some to 
habit; but he would be a bold man who would account by 
such agencies for the differences between a dray- and race- 
horse, a greyhound and bloodhound, a carrier and tumbler 
pigeon. One of the most remarkable features in our domes- 
ticated races is that we see in them adaptation, not indeed to 
the animal's or plant's own good, but to man's use or fancy. 
Some variations useful to him have probably arisen sud- 
denly, or by one step ; many botanists, for instance, believe 
that the fuller's teasel, with its hooks, which cannot be 
rivalled by any mechanical contrivance, is only a variety of 
the wild Dipsacus; and this amount of change may have sud- 
denly arisen in a seedling. So it has probably been with the 
turnspit dog; and this is known to have been the case with 
the ancon sheep. But when we compare the dray-horse and 
race-horse, the dromedary and camel, the various breeds 
of sheep fitted either for cultivated land or mountain pasture, 
with the wool of one breed good for one purpose, and that 
of another breed for another purpose ; when we compare the 
many breeds of dogs, each good for man in different ways ; 
when we compare the game-cock, so pertinacious in battle, 
with other breeds so little quarrelsome, with "everlasting 
layers" which never desire to sit, and with the bantam so 
small and elegant; when we compare the host of agricultural, 
culinary, orchard, and flower-garden races of plants, most 
useful to man at different seasons and for different purposes, 
or so beautiful in his eyes, we must, I think, look further 
than to mere variability. We cannot suppose that all the 


breeds were suddenly produced as perfect and as useful as we 
now see them ; indeed, in many cases, we know that this has 
not been their history. The key is man's power of accumu- 
lative selection : nature gives successive variations ; man adds 
them up in certain directions useful to him. In this sense he 
may be said to have made for himself useful breeds. 

The great power of this principle of selection is not hypo- 
thetical. It is certain that several of our eminent breeders 
have, even within a single lifetime, modified to a large extent 
their breeds of cattle and sheep. In order fully to realise 
what they have done, it is almost necessary to read several 
of the many treatises devoted to this subject, and to inspect 
the animals. Breeders habitually speak of an animal's organi- 
sation as something plastic, which they can model almost as 
they please. If I had space I could quote numerous passages 
to this effect from highly competent authorities. Youatt, 
who was probably better acquainted with the works of agri- 
culturists than almost any other individual, and who was him- 
self a very good judge of animals, speaks of the principle o£ 
selection as "that which enables the agriculturist, not only to 
modify the character of his flock, but to change it altogether. 
It is the magician's wand, by means of which he may summon 
into life whatever form and mould he pleases." Lord Somer- 
ville, speaking of what breeders have done for sheep, says : — 
"It would seem as if they had chalked out upon a wall a form 
perfect in itself, and then had given it existence." In Sax- 
ony the importance of the principle of selection in regard to 
merino sheep is so fully recognised, that men follow it as a 
trade ; the sheep are placed on a table and are studied, like a 
picture by a connoisseur ; this is done three times at intervals 
of months, and the sheep are each time marked and classed, 
so that the very best may ultimately be selected for breeding. 

What English breeders have actually effected is proved by 
the enormous prices given for animals with a good pedigree ; 
and these have been exported to almost every quarter of the 
world. The improvement is by no means generally due to 
crossing different breeds ; all the best breeders are strongly 
opposed to this practice, except sometimes amongst closely 
allied sub-breeds. And when a cross has been made, the 
closest selection is far more indispensable even than in ordi- 


nary cases. If selection consisted merely in separating some 
very distinct variety, and breeding from it. the principle would 
be so obvious as hardly to be worth notice ; but its importance 
consists in the great effect produced by the accumulation in 
one direction, during successive generations, of differences 
absolutely inappreciable by an uneducated eye — differences 
which I for one have vainly attempted to appreciate. Not 
one man in a thousand has accuracy of eye and judgment 
sufficient to become an eminent breeder. If gifted with these 
qualities, and he studies his subject for years, and devotes his 
lifetime to it with indomitable perseverance, he will succeed, 
and may make great improvements ; if he wants any of these 
qualities, he will assuredly fail. Few would readily believe 
in the natural capacity and years of practice requisite to be- 
come even a skilful pigeon-fancier. 

The same principles are followed by horticulturists; but 
the variations are here often more abrupt. No one supposes 
'that our choicest productions have been produced by a single 
variation from the aboriginal stock. We have proofs that 
this has not been so in several cases in which exact records 
have been kept; thus, to give a very trifling instance, the 
steadily increasing size of the common gooseberry may be 
quoted. We see an astonishing improvement in many flor- 
ists' flowers, when the flowers of the present day are com- 
pared with drawings made only twenty or thirty years ago. 
When a race of plants is once pretty well established, the 
seed-raisers do not pick out the best plants, but merely go 
.over their seed-beds, and pull up the "rogues," as they call 
the plants that deviate from the proper standard. With ani- 
mals this kind of selection is, in fact, likewise followed; for 
hardly any one is so careless as to breed from his worst 

In regard to plants, there is another means of observing 
the accumulated effects of selection — namely, by comparing 
the diversity of flowers in the different varieties of the same 
species in the flower-garden ; the diversity of leaves, pods, or 
tubers, or whatever part is valued, in the kitchen-garden, in 
comparison with the flowers of the same varieties ; and the 
diversity of fruit of the same species in the orchard, in com- 
parison with the leaves and flowers of the same set of vari- 


eties. See how different the leaves of the cabbage are, and 
how extremely alike the flowers ; how unlike the flowers of 
the heartsease are, and how alike the leaves; how much the 
fruit of the different kinds of gooseberries differ in size, 
colour, shape, and hairiness, and yet the flowers present very 
slight differences. It is not that the varieties which differ 
largely in some one point do not differ at all in other points; 
this is hardly ever, — I speak after careful observation, — per- 
haps never, the case. The law of correlated variation, the im- 
portance of which should never be overlooked, will ensure 
some differences ; but, as a general rule, it cannot be doubted 
that the continued selection of slight variations, either in the 
leaves, the flowers, or the fruit, will produce races differing 
from each other chiefly in these characters. 

It may be objected that the principle of selection has been 
reduced to methodical practice for scarcely more than three- 
quarters of a century ; it has certainly been more attended to 
of late years, and many treatises have been published on the 
subject; and the result has been, in a corresponding degree, 
rapid and important. But it is very far from true that the 
principle is a modern discovery. I could give several refer- 
ences to works of high antiquity, in which the full impor- 
tance of the principle is acknowledged. In rude and bar- 
barous periods of English history choice animals were often 
imported, and laws were passed to prevent their exportation: 
the destruction of horses under a certain size was ordered, 
and this may be compared to the "roguing" of plants by nur- 
serymen. The principle of selection I find distinctly given in 
an ancient Chinese encyclopaedia. Explicit rules are laid 
down bj some of the Roman classical writers. From pas- 
sages in Genesis, it is clear that the colour of domesticated 
animals was at that early period attended to. Savages now 
sometimes cross their dogs with wild canine animals, to im- 
prove the breed, and they formerly did so, as is attested by 
passages in Pliny. The savages in South Africa match their 
draught cattle by colour, as do some of the Esquimaux their 
teams of dogs. Livingstone states that good domestic breeds 
are highly valued by the negroes in the interior of Africa 
who have not associated with Europeans. Some of these 
facts do not show actual selection, but they show that the 


breeding of domestic animals was carefully attended to in 
ancient times, and is now attended to by the lowest savages. 
It would, indeed, have been a strange fact, had attention 
not been paid to breeding, for the inheritance of good and 
bad qualities is so obvious. 


At the present time, eminent breeders try by methodical 
selection, with a distinct object in view, to make a new strain 
or sub-breed, superior to anything of the kind in the country. 
But, for our purpose, a form of Selection, which may be 
called Unconscious, and which results from every one trying 
to possess and breed from the best individual animals, is more 
important. Thus, a man who intends keeping pointers nat- 
urally tries to get as good dogs as he can, and afterwards 
breeds from his own best dogs, but he has no wish or expec- 
tation of permanently altering the breed. Nevertheless we 
may infer that this process, continued during centuries, would 
improve and modify any breed, in the same way as Bakewell, 
Collins, etc., by this very same process, only carried on more 
methodically, did greatly modify, even during their lifetimes, 
the forms and qualities of their cattle. Slow and insensible 
changes of this kind can never be recognised unless actual 
measurements or careful drawings of the breeds in question 
have been made long ago, which may serve for comparison. 
In some cases, however, unchanged, or but little changed 
individuals of the same breed exist in less civilised districts, 
where the breed has been less improved. There is reason to 
believe that King Charles' spaniel has been unconsciously 
modified to a large extent since the time of that monarch. 
Some highly competent authorities are convinced that the 
setter is directly derived from the spaniel, and has probably 
been slowly altered from it. It is known that the English 
pointer has been greatly changed within the last century, and 
in this case the change has, it is believed, been chiefly effected 
by crosses with the foxhound : but what concerns us is. that 
the change has been effected unconsciously and gradually, and 
yet so effectually, that, though the old Spanish pointer cer- 
tainly came from Spain, Mr. Borrow has not seen, as I am 


informed by him, any native dog in Spain like our pointer. 
By a simple process of selection, and by careful training, 
English racehorses have come to surpass in fleetness and size 
the parent Arabs, so that the latter, by the regulations for the 
Goodwood Races, are favoured in the weights which they 
carry. Lord Spencer and others have shown how the cattle 
of England have increased in weight and in early maturity, 
compared with the stock formerly kept in this country. By 
comparing the accounts given in various old treatises of the 
former and present state of carrier and tumbler pigeons in 
Britain, India, and Persia, we can trace the stages through 
which they have insensibly passed, and come to differ so 
greatly from the rock-pigeon. 

Youatt gives an excellent illustration of the effects of a 
course of selection, which may be considered as unconscious, 
in so far that the breeders could never have expected, or even 
wished, to produce the result which ensued — namely, the pro- 
duction of two distinct strains. The two flocks of Leicester 
sheep kept by Mr. Buckley and Mr. Burgess, as Mr. Youatt 
remarks, "have been purely bred from the original stock of 
Mr. Bakewell for upwards of fifty years. There is not a sus- 
picion existing in the mind of any one at all acquainted with 
the subject, that the owner of either of them has deviated in 
any one instance from the pure blood of Mr. Bakewell's flock, 
and yet the difference between the sheep possessed by these 
two gentlemen is so great that they have the appearance of 
being quite dift'erent varieties." 

If there exist savages so barbarous as never to think of the 
inherited character of the offspring of their domestic animals, 
yet any one animal particularly useful to them, for any 
special purpose, would be carefully preserved during famines 
and other accidents, to which savages are so liable, and such 
choice animals would thus generally leave more offspring 
than the inferior ones ; so that in this case there would be a 
kind of unconscious selection going on. We see the value set 
on animals even by the barbarians of Tierra del Fuego, by 
their killing and devouring their old women, in times of 
dearth, as of less value than their dogs. 

In plants the same gradual process of improvement, 
through the occasional preservation of the best individuals. 


whether or not sufficiently distinct to be ranked at their first 
appearance as distinct varieties, and whether or not two or 
more species or races have become blended together by cross- 
ing, may plainly be recognised in the increased size and beauty 
which we now see in the varieties of the heartsease, rose, 
pelargonium, dahlia, and other plants, when compared with 
the older varieties or with their parent-stocks. No one would 
ever expect to get a first-rate heartsease or dahlia from the 
seed of a wild plant. No one would expect to raise a first- 
rate melting pear from the seed of the wild pear, though he 
might succeed from a poor seedling growing wild, if it had 
come from a garden-stock. The pear though cultivated in 
classical times, appears, from Pliny's description, to have 
been a fruit of very inferior quality. I have seen great sur- 
prise expressed in horticultural works at the wonderful skill 
of gardeners, in having produced such splendid results from 
such poor materials; but the art has been simple, and, as far 
as the final result is concerned, has been followed almost un- 
consciously. It has consisted in always cultivating the best- 
known variety, sowing its seeds, and, when a slightly better 
variety chanced to appear, selecting it, and so onwards. But 
the gardeners of the classical period, who cultivated the best 
pears which they could procure, never thought what splendid 
fruit we should eat; though we owe our excellent fruit in 
some small degree to their having naturally chosen and pre- 
served the best varieties they could anywhere find. 

A large amount of change, thus slowly and unconsciously 
accumulated, explains, as I believe, the well-known fact, that 
in a number of cases we cannot recognise, and therefore do 
not know, the wild parent-stocks of the plants which have 
been longest cultivated in our flower and kitchen gardens. 
If it has taken centuries or thousands of years to improve or 
modify most of our plants up to their present standard of 
usefulness to man, we can understand how it is that neither 
Australia, the Cape of Good Hope, nor any other region in- 
habited by quite uncivilised man, has afforded us a single plant 
worth culture. It is not that these countries, so rich in 
species, do not by a strange chance possess the aboriginal 
stocks of any useful plants, but that the native plants have not 
been improved by continued selection up to a standard of 


perfection comparable with that acquired by the plants in 
countries anciently civilised. 

In regard to the domestic animals kept by uncivilised man, 
it should not be overlooked that they almost always have to 
struggle for their own food, at least during certain seasons. 
And in two countries very differently circumstanced, indi- 
viduals of the same species, having slightly different consti- 
tutions or structure, would often succeed better in the one 
country than in the other; and thus by a process of "natural 
selection," as will hereafter be more fully explained, two sub- 
breeds might be formed. This, perhaps, partly explains why 
the varieties kept by savages, as has been remarked by some 
authors, have more of the character of true species than the 
varieties kept in civilised countries. 

On the view here given of the important part which selec- 
tion by man has played, it becomes at once obvious, how it is 
that our domestic races show adaptation in their structure or 
in their habits to man's wants or fancies. We can, I think, 
further understand the frequently abnormal character of our 
domestic races, and likewise their differences being so great 
in external characters, and relatively so slight in internal 
parts or organs. Man can hardly select, or only with much 
difficulty, any deviation of structure excepting such as is ex- 
ternally visible ; and indeed he rarely cares for what is inter- 
nal. He can never act by selection, excepting on variations 
which are first given to him in some slight degree by nature. 
No man would ever try to make a fantail till he saw a pigeon 
with a tail developed in some slight degree in an unusual 
manner, or a pouter till he saw a pigeon with a crop of some- 
what unusual size ; and the more abnormal or unusual any 
character was when it first appeared, the more likely it would 
be to catch his attention. But to use such an expression as 
trying to make a fantail, is, I have no doubt, in most cases, 
utterly incorrect. The man who first selected a pigeon with 
a slightly larger tail, never dreamed what the descendants of 
that pigeon would become through long-continued, partly 
unconscious and partly methodical, selection. Perhaps the 
parent-bird of all fantails had only fourteen tail-feathers 
somewhat expanded, like the present Java fantail, or like in- 
dividuals of other and distinct breeds, in which as many as 


seventeen tail-feathers have been counted. Perhaps the first 
pouter-pigeon did not inflate its crop much more than the 
turbit now does the upper part of its oesophagus, — a habit 
which is disregarded by all fanciers, as it is not one of the 
points of the breed. 

Nor let it be thought that some great deviation of structure 
would be necessary to catch the fancier's eye: he perceives 
extremely small differences, and it is in human nature to 
fancy any novelty, however slight, in one's own possession. 
Nor must the value which would formerly have been set on 
any slight differences in the individuals of the same species, 
be judged of by the value which is now set on them, after 
several breeds have fairly been established. It is known that 
with pigeons many slight variations now occasionally appear, 
but these are rejected as faults or deviations from the stand- 
ard of perfection in each breed. The common goose has not 
given rise to any marked varieties ; hence the Toulouse and 
the common breed, which differ only in colour, that most 
fleeting of characters, have lately been exhibited as distinct 
at our poultry-shows. 

These views appear to explain what has sometimes been 
noticed — namely, that we know hardly anything about the 
origin or history of any of our domestic breeds. But, in 
fact, a breed, like a dialect of a language, can hardly be said 
to have a distinct origin. A man preserves and breeds from 
an individual with some slight deviation of structure, or takes 
more care than usual in matching his best animals, and thus 
improves them, and the improved animals slowly spread in the 
immediate neighbourhood. But they will as yet hardly have 
a distinct name, and from being only slightly valued, their 
history will have been disregarded. When further improved 
by the same slow and gradual process, they will spread more 
widely, and will be recognised as something distinct and valu- 
able, and will then probably first receive a provincial name. 
In semi-civilised countries, with little free comnnmication, 
the spreading of a new sub-breed would be a slow process. 
As soon as the points of value are once acknowledged, the 
principle, as I have called it, of unconscious selection will 
always tend, — perhaps more at one period than at another, as 
the breed rises or falls in fashion, — perhaps more in one dis- 


trict than in another, according to the state of civilisation of 
the inhabitants, — slowly to add to the characteristic features 
of the breed, whatever they may be. But the chance will be 
infinitely small of any record having been preserved of such 
slow, varying, and insensible changes. 


I will now say a few words on the circumstances, favour- 
able, or the reverse, to man's power of selection. A high de- 
gree of variability is obviously favourable, as freely giving 
the materials for selection to work on; not that mere indi- 
vidual differences are not amply sufficient, with extreme care, 
to allow of the accumulation of a large amount of modifica- 
tion in almost any desired direction. But as variations mani- 
festly useful or pleasing to man appear only occasionally, the 
chance of their appearance will be much increased by a large 
number of individuals being kept. Hence, number is of the 
highest importance for success. On this principle Marshall 
formerly remarked, with respect to the sheep of parts of 
Yorkshire, "as they generally belong to poor people, and are 
mostly in small lots, they never can be improved." On the 
other hand, nurserymen, from keeping large stocks of the 
same plant, are generally far more successful than amateurs 
in raising new and valuable varieties. A large number of 
individuals of an animal or plant can be reared only where 
the conditions for its propagation are favourable. When the 
individuals are scanty, all will be allowed to breed, whatever 
their quality may be, and this will effectually prevent selec- 
tion. But probably the most important element is that the 
animal or plant should be so highly valued by man, that the 
closest attention is paid to even the slightest deviations in its 
qualities or structure. Unless such attention be paid nothing 
can be effected. I have seen it gravely remarked, that it was 
most fortunate that the strawberry began to vary just when 
gardeners began to attend to this plant. No doubt the straw- 
berry had always varied since it was cultivated, but the slight 
varieties had been neglected. As soon, however, as gar- 
deners picked out individual plants with slightly larger, ear- 
lier, or better fruit, and raised seedlings from them, and again 


picked out the best secdlin.^-s and bred from them, then (with 
some aid by crossing distinct species) those many admirable 
varieties of the strawberry were raised which have appeared 
during the last half-century. 

With animals, facility in preventing crosses is an important 
element in the formation of new races, — at least, in a country 
which is already stocked with other races. In this respect 
enclosure of the land plays a part. Wandering savages or 
the inhabitants of open plains rarely possess more than one 
breed of the same species. Pigeons can be mated for life, and 
this is a great convenience to the fancier, for thus many races 
may be improved and kept true, though mingled in the same 
aviary ; and this circumstance must have largely favoured the 
formation of new breeds. Pigeons, I may add, can be propa- 
gated in great numbers and at a very quick rate, and inferior 
birds may be freely rejected, as when killed they serve for 
food. On the other hand, cats, from their nocturnal rambling 
habits, cannot be easily matched, and, although so much 
valued by women and children, we rarely see a distinct breed 
long kept up ; such breeds as we do sometimes see are almost 
always imported from some other country. Although I do 
not doubt that some domestic animals vary less than others, 
yet the rarity or absence of distinct breeds of the cat, the 
donkey, peacock, goose, &c., may be attributed in main part 
to selection not having been brought into play : in cats, from 
the difficulty in pairing them ; in donkeys, from only a few 
being kept by poor people, and little attention paid to their 
breeding; for recently in certain parts of Spain and of the 
United States this animal has been surprisingly modified and 
improved by careful selection ; in peacocks, from not being 
very easily reared and a large stock not kept; in geese, from 
being valuable only for two purposes, food and feathers, and 
more especially from no pleasure having been felt in the dis- 
play of distinct breeds ; but the goose, under the conditions to 
which it is exposed when domesticated, seems to have a sin- 
gularly inflexible organisation, though it has varied to a 
slight extent, as I have elsewhere described. 

Some authors have maintained that the amount of variation 
in our domestic productions is soon reached, and can never 
afterwards be exceeded. It would be somewhat rash to as- 


sert that the limit has been attained in any one case ; for al- 
most all our animals and plants have been greatly improved in 
many w^ays within a recent period ; and this implies variation. 
It vi^ould be equally rash to assert that characters now in- 
creased to their usual limit, could not, after remaining fixed 
for many centuries, again vary under new conditions of life. 
No doubt, as Mr. Wallace has remarked with much truth, a 
limit will be at last reached. For instance, there must be a 
limit to the fleetness of any terrestrial animal, as this will 
be determined by the friction to be overcome, the weight of 
body to be carried, and the power of contraction in the mus- 
cular fibres. But what concerns us is that the domestic vari- 
eties of the same species differ from each other in almost 
every character, which man has attended to and selected, 
more than do the distinct species of the same genera. Isi- 
dore Geoffroy St. Hilaire has proved this in regard to size, 
and so it is with colour and probably with the length of hair. 
With respect to fleetness, which depends on many bodily char- 
acters, Eclipse was far fleeter, and a dray-horse is incom- 
parably stronger than any two natural species belonging to 
the same genus. So with plants, the seeds of the different 
varieties of the bean or maize probably differ more in size, 
than do the seeds of the distinct species in any one genus in 
the same two families. The same remark holds good in re- 
gard to the fruit of the several varieties of the plum, and still 
more strongly with the melon, as well as in many other anal- 
ogous cases. 

To sum up on the origin of our domestic races of animals 
and plants. Changed conditions of life are of the highest 
importance in causing variability, both by acting directly on 
the organisation, and indirectly by affecting the reproductive 
system. It is not probable that variability is an inherent and 
necessary contingent, under all circumstances. The greater 
or less force of inheritance and reversion determine whether 
variations shall endure. Variability is governed by many 
unknown laws, of which correlated growth is probably the 
most important. Something, but how much we do not know, 
may be attributed to the definite action of the conditions of 
life. Some, perhaps a great, effect may be attributed to the 
increased use or disuse of parts. The final result is thus 


rendered infinitely complex. In some cases the intercrossing 
of aboriginally distinct species appears to have played an im- 
portant part in the origin of our breeds. When several 
breeds have once been formed in any country, their occa- 
sional intercrossing, with the aid of selection, has, no doubt, 
largely aided in the formation of new sub-breeds ; but the im- 
portance of crossing has been much exaggerated, both in re- 
gard to animals and to those plants which are propagated by 
seed. With plants which are temporarily propagated by cut- 
tings, buds, &c., the importance of crossing is immense ; for 
the cultivator may here disregard the extreme variability 
both of hybrids and of mongrels, and the sterility of hybrids; 
but plants not propagated by seed are of little importance to 
us, for their endurance is only temporary. Over all these 
causes of Change, the accumulative action of Selection, 
whether applied methodically and quickly, or unconsciously 
and slowly but more efficiently, seems to have been the pre- 
dominant Power. 


Variation Under Nature 

Variability — Individual differences — Doubtful species — Wide ranging, 
much diffused, and common species, vary most — Species of the 
larger genera in each country vary more frequently than the 
species of the smaller genera — Many of the species of the larger 
genera resemble varieties in being very closely, but unequally, 
related to each other, and in having restricted ranges. 


EFORE applying the principles arrived at in the last 
chapter to organic beings in a state of nature, we must 
briefly discuss whether these latter are subject to any 
variation. To treat this subject properly, a long catalogue of 
dry facts ought to be given ; but these I shall reserve for a 
future work. Nor shall I here discuss the various definitions 
which have been given of the term species. No one defini- 
tion has satisfied all naturalists; yet every naturalist knows 
vaguely what he means when he speaks of a species. Gen- 
erally the term includes the unknown element of a distinct 
act of creation. The term "variety" is almost equally diffi- 
cult to define ; but here community of descent is almost uni- 
versally implied, though it can rarely be proved. We have 
also what are called monstrosities ; but they graduate into 
varieties. By a monstrosity I presume is meant some consid- 
erable deviation of structure, generally injurious, or not use- 
ful to the species. Some authors use the term "variation" in 
a technical sense, as implying a modification directly due to 
the physical conditions of life; and "variations" in this sense 
are supposed not to be inherited; but who can say that the 
dwarfed condition of shells in the brackish waters of the 
Baltic, or dwarfed plants on Alpine summits, or the thicker 
fur of an animal from far northwards, would not in some 
cases be inherited for at least a few generations ? and in this 
case I presume that the form would be called a variety. 
It may be doubted whether sudden and considerable devi- 



ations of structure such as we occasionally see in our domes- 
tic productions, more especially with plants, are ever perma- 
nently propagated in a state of nature. Almost every part 
of every organic being is so beautifully related to its complex 
conditions of life that it seems as improbable that any part 
should have been suddenly produced perfect, as that a com- 
plex machine should have been invented by man in a perfect 
state. Under domestication monstrosities sometimes occur 
which resemble normal structures in widely different animals. 
Thus pigs have occasionally been born with a sort of pro- 
boscis, and if any wild species of the same genus had nat- 
urally possessed a proboscis, it might have been argued that 
this had appeared as a monstrosity ; but I have as yet failed 
to find, after diligent search, cases of monstrosities resem- 
bling normal structures in nearly allied forms, and these alone 
bear on the question. If monstrous forms of this kind ever 
do appear in a state of nature and are capable of reproduc- 
tion (which is not always the case), as they occur rarely and 
singly, their preservation would depend on unusually favour- 
able circumstances. They would, also, during the first and 
succeeding generations cross with the ordinary form, and 
thus their abnormal character would almost inevitably be lost. 
But I shall have to return in a future chapter to the pres- 
ervation and perpetuation of single or occasional variations. 


The many slight differences which appear in the oft'spring 
from the same parents, or which it may be presumed have 
thus arisen, from being observed in the individuals of the 
same species inhabiting the same confined locality, may be 
called individual dift'erences. No one supposes that all the 
individuals of the same species are cast in the same actual 
mould. These individual differences are of the highest im- 
portance for us, for they are often inherited, as must be 
familiar to every one ; and they thus aft'ord materials for 
natural selection to act on and accumulate, in the same man- 
ner as man accumulates in any given direction individual dif- 
ferences in his domesticated productions. These individual 
differences generally aft'ect what naturalists consider unim- 


portant parts ; but I could show by a long catalogue of facts, 
that parts which must be called important, whether viewed 
under a physiological or classificatory point of view, some- 
times vary in the individuals of the same species. I am con- 
vinced that the most experienced naturalist would be sur- 
prised at the number of the cases of variability, even in im- 
portant parts of structure, which he could collect on good 
authority, as I have collected, during a course of years. It 
should be remembered that systematists are far from being 
pleased at finding variability in important characters, and that 
there are not many men who will laboriously examine inter- 
nal and important organs, and compare them in many speci- 
mens of the same species. It would never have been expected 
that the branching of the main nerves close to the great cen- 
tral ganglion of an insect would have been variable in the 
same species ; it might have been thought that changes of 
this nature could have been effected only by slow degrees; 
yet Sir J. Lubbock has shown a degree of variability in these 
main nerves in Coccus, which may almost be compared to the 
irregular branching of the stem of a tree. This philosoph- 
ical naturalist, I may add, has also shown that the muscles in 
the larvae of certain insects are far from uniform. Authors 
sometimes argue in a circle when they state that important 
organs never vary; for these same authors practically rank 
those parts as important (as some few naturalists have hon- 
estly confessed) which do not vary; and, under this point of 
view, no instance will ever be found of an important part 
varying; but under any other point of view many instances 
assuredly can be given. 

There is one point connected with individual differences, 
which is extremely perplexing : I refer to those genera which 
have been called "protean" or "polymorphic," in which the 
species present an inordinate amount of variation. With re- 
spect to many of these forms, hardly two naturalists agree 
whether to rank them as species or as varieties. We may 
instance Rubus, Rosa, and Hieracium amongst plants, several 
genera of insects and of Brachiopod shells. In most poly- 
morphic genera some of the species have fixed and definite 
characters. Genera which are polymorphic in one country 
seem to be, with a few exceptions, polymorphic in other coun- 


tries, and likewise, judging from Brachiopod shells, at former 
periods of time. These facts are very perplexing, for they 
seem to show that this kind of variability is independent of 
the conditions of life. I am inclined to suspect that we see, 
at least in some of these polymorphic genera, variations which 
are of no service or disservice to the species, and which con- 
sequently have not been seized on and rendered definite by 
natural selection, as hereafter to be explained. 

Individuals of the same species often present, as is known 
to every one, great differences of structure, independently 
of variation, as in the two sexes of various animals, in 
the two or three castes of sterile female or workers amongst 
insects, and in the immature and larval states of many of 
the lower animals. 

There are, also, cases of dimorphism and trimorphism, 
both with animals and plants. Thus, Mr. Wallace, who 
has lately called attention to the subject, has shown 
that the females of certain species of butterflies, in the Ma- 
layan archipelago, regularly appear under two or even three 
conspicuously distinct forms, not connected by intermediate 
varieties. Fritz Miiller has described analogous but more 
extraordinary cases with the males of certain Brazilian 
Crustaceans: thus, the male of a Tanais regularly occurs 
under two distinct forms; one of these has strong and dif- 
ferently shaped pincers, and the other has antennae much 
more abundantly furnished with smelling-hairs. Although 
in most of these cases, the two or three forms, both with 
animals and plants, are not now connected by intermediate 
gradations, it is probable that they were once thus connected. 
Mr. Wallace, for instance, describes a certain butterfly which 
presents in the same island a great range of varieties con- 
nected by intermediate links, and the extreme links of the 
chain closely resemble the two forms of an allied dimorphic 
species inhabiting another part of the Malay archipelago. 
Thus also with ants, the several worker-castes are generally 
quite distinct; but in some cases, as we shall hereafter see, 
the castes are connected together by finely graduated varie- 
ties. So it is, as I have myself observed, with some dimor- 
phic plants. It certainly at first appears a highly remarkable 
fact that the same female butterfly should have the power 


of producing at the same time three distinct female forms 
and a male ; and that an hermaphrodite plant should produce 
from the same seed-capsule three distinct hermaphrodite 
forms, bearing three different kinds of females and three or 
even six different kinds of males. Nevertheless these cases 
are only exaggerations of the common fact that the female 
produces offspring of two sexes which sometimes differ from 
each other in a wonderful manner. 


The forms which possess in some considerable degree the 
character of species, but which are so closely similar to other 
forms, or are so closely linked to them by intermediate gra- 
dations, that naturalists do not like to rank them as distinct 
species, are in several respects the most important for us. 
We have every reason to believe that many of these doubtful 
and closely allied forms have permanently retained their 
characters for a long time ; for as long, as far as we know, 
as have good and true species. Practically, when a naturalist 
can unite by means of intermediate links any two forms, he 
treats the one as a variety of the other; ranking the most 
common, but sometimes the one first described, as the spe- 
cies, and the other as the variety. But cases of great diffi- 
culty, which I will not here enumerate, sometimes arise in 
deciding whether or not to rank one form as a variety of 
another, even when they are closely connected by interme- 
diate links; nor will the commonly-assumed hybrid nature 
of the intermediate forms always remove the difficulty. In 
very many cases, however, one form is ranked as a variety 
of another, not because the intermediate links have actually 
been found, but because analogy leads the observer to sup- 
pose either that they do now somewhere exist, or may for- 
merly have existed; and here a wide door for the entry of 
doubt and conjecture is opened. 

Hence, in determining whether a form should be ranked 
as a species or a variety, the opinion of naturalists having 
sound judgment and wide experience seems the only guide to 
follow. We must, however, in many cases, decide by a ma- 
jority of naturalists, for few well-marked and well-known 


varieties can be named which have not been ranked as spe- 
cies by at least some competent judges. 

That varieties of this doubtful nature are far from un- 
common cannot be disputed. Compare the several floras of 
Great Britain, of France, or of the United States, drawn up 
by different botanists, and see what a surprising number of 
forms have been ranked by one botanist as good species, 
and by another as mere varieties. Mr. H. C. Watson, to 
whom I lie under deep obligation for assistance of all kinds, 
has marked for me 182 British plants, which are generally 
considered as varieties, but which have all been ranked by 
botanists as species; and in making this list he has omitted 
many trifling varieties, but which nevertheless have been 
ranked by some botanists as species, and he has entirely 
omitted several highly polymorphic genera. Under genera, 
including the most polymorphic forms, Mr. Babington gives 
251 species, whereas Mr. Bentham gives only 112, — a differ- 
ence of 139 doubtful forms ! Amongst animals which unite 
for each birth, and which are highly locomotive, doubtful 
forms, ranked by one zoologist as a species and by another 
as a variety, can rarely be found within the same country, 
but are common in separated areas. How many of the birds 
and insects in North America and Europe, which differ very 
slightly from each other, have been ranked by one eminent 
naturalist as undoubted species, and by another as varieties, 
or, as they are often called, geographical races ! Mr. Wallace, 
in several valuable papers on the various animals, especially 
on the Lepidoptera, inhabiting the islands of the great Ma- 
layan archipelago, shows that they may be classed under four 
heads, namely, as variable forms, as local forms, as geo- 
graphical races or sub-species, and as true representative 
species. The first or variable forms vary much within tho 
limits of the same island. The local forms are moderately 
constant and distinct in each separate island; but when all 
from the several islands are compared together, the differ- 
ences are seen to be so slight and graduated, that it is im- 
possible to define or describe them, though at the same time 
the extreme forms are sufficiently distinct. The geo- 
graphical races or sub-species are local forms completely 
fixed and isolated; but as they do not differ from each other 


by strongly marked and important characters, "there is no 
possible test but individual opinion to determine which of 
them shall be considered as species and which as varieties." 
Lastly, representative species fill the same place in the nat' 
ural economy of each island as do the local forms and sub- 
species; but as they are distinguished from each other by a 
greater amount of difference than that between the local 
forms and sub-species, they are almost universally ranked 
by naturalists as true species. Nevertheless, no certain cri- 
terion can possibly be given by which variable forms, local 
forms, sub-species, and representative species can be 

Many years ago, when comparing, and seeing others com- 
pare, the birds from the closely neighbouring islands of the 
Galapagos archipelago, one with another, and with those 
from the American mainland, I was much struck how entirely 
vague and arbitrary is the distinction between species and 
varieties. On the islets of the little Madeira group there are 
many insects which are characterised as varieties in Mr. 
Wollaston's admirable work, but which would certainly 
be ranked as distinct species by many entomologists. Even 
Ireland has a few animals, now generally regarded as 
varieties, but which have been ranked as species by some 
zoologists. Several experienced ornithologists consider our 
British red grouse as only a strongly-marked race of Nor- 
wegian species, whereas the greater number rank it as an 
undoubted species peculiar to Great Britain. A wide dis- 
tance between the homes of two doubtful forms leads many 
naturalists to rank them as distinct species ; but what dis- 
tance, it has been well asked, will suffice; if that between 
America and Europe is ample, will that between Europe and 
the Azores, or Madeira, or the Canaries, or between the sev- 
eral islets of these small archipelagos, be sufficient? 

Mr. B. D. Walsh, a distinguished entomologist of the 
United States, has described what he calls Phytophagic 
varieties and Phytophagic species. Most vegetable-feeding 
insects live on one kind of plant or on one group of plants; 
some feed indiscriminately on many kinds, but do not in 
consequence vary. In several cases, however, insects found 
living on different plants, have been observed by Mr. Walsh 


to present in their larval or mature state, or in both states, 
slight, though constant differences in colour, size, or in the 
nature of their secretions. In some instances the males 
alone, in other instances both males and females, have been 
observed thus to differ in a slight degree. When the differ- 
ences are rather more strongly marked, and when both 
sexes and all ages are affected, the forms are ranked by all 
entomologists as good species. But no observer can deter- 
mine for another, even if he can do so for himself, which of 
these Phytophagic forms ought to be called species and 
which varieties. Mr. Walsh ranks the forms which it may 
be supposed would freely intercross, as varieties ; and those 
which appear to have lost this power, as species. As the 
differences depend on the insects having long fed on distinct 
plants, it cannot be expected that intermediate links connect- 
ing the several forms should now be found. The naturalist 
thus loses his best guide in determining whether to rank 
doubtful forms as varieties or species. This likewise neces- 
sarily occurs with closely allied organisms, which inhabit 
distinct continents or islands. When, on the other hand, 
an animal or plant ranges over the same continent, or in- 
habits many islands in the same archipelago, and presents 
different forms in the different areas, there is always a good 
chance that intermediate forms will be discovered which will 
link together the extreme states ; and these are then degraded 
to the rank of varieties. 

Some few naturalists maintain that animals never present 
varieties; but then these same naturalists rank the slightest 
difference as of specific value ; and when the same identical 
form is met with in two distinct countries, or in two geologi- 
cal formations, they believe that two distinct species are hid- 
den under the same dress. The term species thus comes to 
be a mere useless abstraction, implying and assuming a sep- 
arate act of creation. It is certain that many forms, consid- 
ered by highly-competent judges to be varieties, resemble 
species so completely in character, that they have been thus 
ranked by other highly-competent judges. But to discuss 
whether they ought to be called species or varieties, before 
any definition of these terms has been generally accepted, is 

vainly to beat the air. 

c— lie XI 


Many of the cases of strongly-marked varieties or doubtful 
species well deserve consideration ; for several interesting 
lines of argument, from geographical distribution, analogical 
variation, hybridism, &c., have been brought to bear in the 
attempt to determine their rank ; but space does not here per- 
mit me to discuss them. Close investigation, in many cases, 
v^^ill no doubt bring naturalists to agree how to rank doubt- 
ful forms. Yet it must be confessed that it is in the best 
known countries that we find the greatest number of them. 
I have been struck with the fact, that if any animal or plant 
in a state of nature be highly useful to man, or from any 
cause closely attracts his attention, varieties of it will almost 
universally be found recorded. These varieties, moreover, 
will often be ranked by some authors as species. Look at the 
common oak, how closely it has been studied; yet a German 
author makes more than a dozen species out of forms, which 
are almost universally considered by other botanists to be 
varieties ; and in this country the highest botanical authori- 
ties and practical men can be quoted to show that the sessile 
and pedunculated oaks are either good and distinct species or 
mere varieties. 

I may here allude to a remarkable memoir lately published 
by A. de CandoUe, on the oaks of the whole world. No one 
ever had more ample materials for the discrimination of the 
species, or could have worked on them with more zeal and 
sagacity. He first gives in detail all the many points of struc- 
ture which vary in the several species, and estimates numeri- 
cally the relative frequency of the variations. He specifies 
above a dozen characters which may be found varying even 
on the same branch, sometimes according to age or develop- 
ment, sometimes without any assignable reason. Such char- 
acters are not of course of specific value, but they are, as Asa 
Gray has remarked in commenting on this memoir, such as 
generally enter into specific definitions. De Candolle then 
goes on to say that he gives the rank of species to the forms 
that differ by characters never varying on the same tree, and 
never found connected by intermediate states. After this 
discussion, the result of so much labour, he emphatically re- 
marks: "They are mistaken, who repeat that the greater 
part of our species are clearly limited, and that the doubtful 


species are in a feeble minority. This seemed to be true, so 
long as a genus was imperfectly known, and its species were 
founded upon a few specimens, that is to say, were pro- 
visional. Just as we come to know them better, intermediate 
forms flow in, and doubts as to specific limits augment." He 
also adds that it is the best known species which present the 
greatest number of spontaneous varieties and sub-varieties. 
The Quercus robur has twenty-eight varieties, all of which, 
excepting six, are clustered round three sub-species, namely, 
Q. pedunculata sessiliflora, and pubescens. The forms which 
connect these three sub-species are comparatively rare; and, 
as Asa Gray again remarks, if these connecting forms which 
are now rare, were to become wholly extinct, the three sub- 
species would hold exactly the same relation to each other, as 
do tlie four or five provisionally admitted species which 
closely surround the typical Quercus robur. Finally. De 
Candolle admits that out of the 300 species, which will be 
enumerated in his Prodromus as belonging to the oak family, 
at least two-thirds are provisional species, that is, are not 
known strictly to fulfil the definition above given of a true 
species. It should be added that De Candolle no longer be- 
lieves that species are immutable creations, but concludes 
that the derivative theory is the most natural one, "and the 
most accordant with the known facts in palaeontology, geo- 
graphical botany and zoology, of anatomical structure and 

When a young naturalist commences the study of a group 
of organisms quite unknown to him, he is at first much per- 
plexed in determining what differences to consider as specific, 
and what as varietal; for he knows nothing of the amount 
and kind of variation to which the group is subject; and this 
shows, at least, how very generally there is some variation. 
But if he confine his attention to one class within one country, 
he will soon make up his mind how to rank most of the doubt- 
ful forms. His general tendency will be to make many 
species, for he will become impressed, just like the pigeon or 
poultry fancier before alluded to, with the amount of differ- 
ence in the forms which he is continually studying; and he 
has little general knowledge of analogical variation in other 
groups and in other countries, by which to correct his first 


impressions. As he extends the range of his observations, he 
will meet with more cases of difliculty ; for he will encounter 
a greater number of closely-allied forms. But if his observa- 
tions be widely extended, he will in the end generally be able 
to make up his own mind; but he will succeed in this at the 
expense of admitting much variation, — and the truth of this 
admission will often be disputed by other naturalists. When 
he comes to study allied forms brought from countries not 
now continuous, in which case he cannot hope to find inter- 
mediate links, he will be compelled to trust almost entirely to 
analogy, and his difificulties will rise to a climax. 

Certainly no clear line of demarcation has as yet been 
drawn between species and sub-species — that is, the forms 
which in the opinion of some naturalists come very near to, 
but do not quite arrive at, the rank of species: or, again, 
between sub-species and well-marked varieties, or between 
lesser varieties and individual differences. These differences 
blend into each other by an insensible series ; and a series 
impresses the mind with the idea of an actual passage. 

Hence I look at individual differences, though of small 
interest to the systematist, as of the highest importance for 
us, as being the first steps towards such slight varieties as 
are barely thought worth recording in works on natural his- 
tory. And I look at varieties which are in any degree more 
distinct and permanent, as steps towards more strongly- 
marked and permanent varieties; and at the latter, as lead- 
ing to sub-species, and then to species. The passage from 
one stage of difference to another may, in many cases, be 
the simple result of the nature of the organism and of the 
different physical conditions to which it has long been ex- 
posed ; but with respect to the more important and adaptive 
characters, the passage from one stage of difference to an- 
other, may be safely attributed to the cumulative action of 
natural selection, hereafter to be explained, and to the effects 
of the increased use or disuse of parts. A well-marked vari- 
ety may therefore be called an incipient species ; but whether 
this belief is justifiable must be judged by the weight of the 
various facts and considerations to be given throughout this 

It need not be supposed that all varieties or incipient 


species attain the rank of species. They may become extinct, 
or they may endure as varieties for very long periods, as 
has been shown to be the case by Mr. Wollaston with the 
varieties of certain fossil land-shells in Madeira, and with 
plants by Gaston de Saporta. If a variety were to flourish 
so as to exceed in numbers the parent species, it w'ould then 
rank as the species, and the species as the variety ; or it 
might come to supplant and exterminate the parent species; 
or both might co-exist, and both rank as independent species. 
But we shall hereafter return to this subject. 

From these remarks it will be seen that I look at the term 
species as one arbitrarily given, for the sake of convenience, 
to a set of individuals closely resembling each other, and 
that it does not essentially differ from the term variety, which 
is given to less distinct and more fluctuating forms. The 
term variety, again, in comparison with mere individual dif- 
ferences, is also applied arbitrarily, for convenience' sake. 



Guided by theoretical considerations, I thought that some 
interesting results might be obtained in regard to the nature 
and relations of the species which vary most, by tabulating 
all the varieties in several well-worked floras. At first this 
seemed a simple task ; but Mr. H. C. Watson, to whom I am 
much indebted for valuable advice and assistance on this 
subject, soon convinced me that there were many difficulties, 
as did subsequently Dr. Hooker, even in stronger terms. I 
shall reserve for a future work the discussion of these diffi- 
culties, and the tables of the proportional numbers of the 
varying species. Dr. Hooker permits me to add that after 
having carefully read my manuscript, and examined the 
tables, he thinks that the following statements are fairly well 
established. The whole subject, however, treated as it neces- 
sarily here is with much brevity, is rather perplexing, and 
allusions cannot be avoided to the "struggle for existence," 
"divergence of character," and other questions, hereafter to 
be discussed. 

Alphonse de Candolle and others have shown that plants 


which have very v^^ide ranges generally present varieties ; 
and this might have been expected, as they are exposed to 
diverse physical conditions, and as they come into competi- 
tion (which, as we shall hereafter see, is an equally or more 
important circumstance) with different sets of organic beings. 
But my tables further show that, in any limited country, the 
species which are the most common, that is abound most in 
individuals, and the species which are most widely diffused 
within their own country (and this is a different considera- 
tion from wide range, and to a certain extent from com- 
monness), oftenest give rise to varieties sufficiently well- 
marked to have been recorded in botanical works. Hence 
it is the most flourishing, or, as they may be called, the 
dominant species, — those which range widely, are the most 
diffused in their own country, and are the most numerous 
in individuals, — which oftenest produce well-marked varie- 
ties, or, as I consider them, incipient species. And this, per- 
haps, might have been anticipated; for, as varieties, in order 
to become in any degree permanent, necessarily have to 
struggle with the other inhabitants of the country, the spe- 
cies which are already dominant will be the most likely to 
yield offspring, which, though in some slight degree modi- 
fied, still inherit those advantages that enabled their parents 
to become dominant over their compatriots. In these re- 
marks on predominance, it should be understood that refer- 
ence is made only to the forms which come into competition 
with each other, and more especially to the members of the 
same genus or class having nearly similar habits of life. 
With respect to the number of individuals or commonness 
of species, the comparison of course relates only to the 
members of the same group. One of the higher plants may 
be said to be dominant if it be more numerous in individuals 
and more widely diffused than the other plants of the same 
country, which live under nearly the same conditions. A 
plant of this kind is not the less dominant because some 
conferva inhabiting the water or some parasitic fungus is 
infinitely more numerous in individuals, and more widely 
diffused. But if the conferva or parasitic fungus exceeds 
its allies in the above respects, it will then be dominant 
within its own class. 





If the plants inhabiting a country, as described in any 
Flora, be divided into two equal masses, all those in the 
larger genera (i.e., those including many species) being 
placed on one side, and all those in the smaller genera on 
the other side, the former will be found to include a some- 
what larger number of the very common and much diffused 
or dominant species. This might have been anticipated; for 
the mere fact of many species of the same genus inhabiting 
any country, shows that there is something in the organic 
or inorganic conditions of that country favourable to the 
genus; and, consequently, we might have expected to have 
found in the larger genera, or those including many species, 
a larger proportional number of dominant species. But so 
many causes tend to obscure this result, that I am surprised 
that my tables show even a small majority on the side of 
the larger genera. I will here allude to only two causes of 
obscurity. Fresh-water and salt-loving plants generally 
have very wide ranges and are much diffused, but this seems 
to be connected with the nature of the stations inhabited by 
them, and has little or no relation to the size of the genera 
to which the species belong. Again, plants low in the scale 
of organisation are generally much more widely diffused 
than plants higher in the scale ; and here again there is no 
close relation to the size of the genera. The cause of lowly- 
organised plants ranging widely will be discussed in our 
chapter on Geographical Distribution. 

From looking at species as only strongly-marked and well- 
defined varieties, I was led to anticipate that the species of 
the larger genera in each country would oftener present 
varieties, than the species of the smaller genera ; for wher- 
ever many closely related species (i.e., species of the same 
genus) have been formed, many varieties or incipient spe- 
cies ought, as a general rule, to be now forming. Where 
m.any large trees grow, we expect to find saplings. Where 
many species of a genus have been formed through varia- 
tion, circumstances have been favourable for variation; and 


hence we might expect that the circumstances would gener- 
ally be still favourable to variation. On the other hand, if 
we look at each species as a special act of creation, there is 
no apparent reason why more varieties should occur in a 
group having many species, than in one having few. 

To test the truth of this anticipation I have arranged the 
plants of twelve countries, and the coleopterous insects of 
tVi'O districts, into two nearly equal masses, the species of 
the larger genera on one side, and those of the smaller genera 
on the other side, and it has invariably proved to be the case 
that a larger proportion of the species on the side of the 
larger genera presented varieties, than on the side of the 
smaller genera. Moreover, the species of the large genera 
which present any varieties, invariably present a larger 
average number of varieties than do the species of the small 
genera. Both these results follow when another division is 
made, and when all the least genera, with from only one to 
four species, are altogether excluded from the tables. These 
facts are of plain signification on the view that species are 
only strongly-marked and permanent varieties; for wherever 
many species of the same genus have been formed, or where, 
if we may use the expression, the manufactory of species 
has been active, we ought generally to find the manufactory 
still in action, more especially as we have every reason to 
believe the process of manufacturing new species to be a 
slow one. And this certainly holds true, if varieties be 
looked at as incipient species ; for my tables clearly show as 
a general rule that, wherever many species of a genus have 
been formed, the species of that genus present a number of 
varieties, that is of incipient species, beyond the average. 
It is not that all large genera are now varying much, and 
are thus increasing in the number of their species, or that 
no small genera are now varying and increasing; for if this 
had been so, it would have been fatal to my theory: inas- 
much as geology plainly tells us that small genera have in 
the lapse of time often increased greatly in size; and that 
large genera have often come to their maxima, decline, and 
disappeared. All that we want to show is, that, where many 
species of a genus have been formed, on an average many 
are still forming; and this certainly holds good. 






There are other relations between the species of large 
genera and their recorded varieties which deserve notice. We 
have seen that there is no infallible criterion by which to 
distinguish species and well-marked varieties ; and when in- 
termediate links have not been found between doubtful 
forms, naturalists are compelled to come to a determination 
by the amount of difference between them, judging by anal- 
ogy whether or not the amount suffices to raise one or both 
to the rank of species. Hence the amount of difference is 
one very important criterion in settling whether two forms 
should be ranked as species or varieties. Now Fries has 
remarked in regard to plants, and Westwood in regard to 
insects, that in large genera the amount of difference be- 
tween the species is often exceedingly small. I have en- 
deavoured to test this numerically by averages, and, as far 
as my imperfect results go, they confirm the view. I have 
also consulted some sagacious and experienced observers, 
and, after deliberation, they concur in this view. In this 
respect, therefore, the species of the larger genera resemble 
varieties, more than do the species of the smaller genera. 
Or the case may be put in another way, and it may be said, 
that in the larger genera, in which a number of varieties or 
incipient species greater than the average are now manu- 
facturing, many of the species already manufactured still to 
a certain extent resemble varieties, for they differ from each 
other by less than the usual amount of difference. 

Moreover, the species of the larger genera are related to 
each other, in the same manner as the varieties of any one 
species are related to each other. No naturalist pretends 
that all the species of a genus are equally distinct from each 
other ; they may generally be divided into sub-genera, or sec- 
tions, or lesser groups. As Fries has well remarked, little 
groups of species are generally clustered like satellites 
around other species. And what are varieties but groups of 
forms, unequally related to each other, and clustered round 


certain forms — that is, round their parent-species? Un- 
doubtedly there is one most important point of difference 
between varieties and species; namely, that the amount of 
difference between varieties, when compared with each other 
or with their parent-species, is much less than that between 
the species of the same genus. But when we come to discuss 
the principle, as I call it, of Divergence of Character, we 
shall see how this may be explained, and how the lesser dif- 
ferences between varieties tend to increase into the greater 
differences between species. 

There is one other point which is worth notice. Varieties 
generally have much restricted ranges : this statement is in- 
deed scarcely more than a truism, for, if a variety were 
found to have a wider range than that of its supposed parent- 
species, their denominations would be reversed. But there 
is reason to believe that the species which are very closely 
allied to other species, and in so far resemble varieties, often 
have much restricted ranges. For instance, Mr. H. C. Wat- 
son has marked for me in the well-sifted London Catalogue 
of plants (4th edition) 63 plants which are therein ranked 
as species, but which he considers as so closely allied to other 
species as to be of doubtful value: these 63 reputed species 
range on an average over 6 '9 of the provinces into which 
Mr. Watson has divided Great Britain. Now, in this same 
Catalogue, 53 acknowledged varieties are recorded, and these 
range over 7 "j provinces ; whereas, the species to which these 
varieties belong range over 14 "3 provinces. So that the ac- 
knowledged varieties have nearly the same restricted aver- 
age range, as have the closely allied forms, marked for me 
by Mr. Watson as doubtful species, but which are almost 
universally ranked by British botanists as good and true 


Finally, varieties cannot be distinguished from species, — 
except, first, by the discovery of intermediate linking forms ; 
and, secondly, by a certain indefinite amount of difference 
between them; for two forms, if differing very little, are 
generally ranked as varieties, notwithstanding that they 
cannot be closely connected; but the amount of difference 


considered necessary to give to any two forms the rank of 
species cannot be defined. In genera having more than the 
average number of species in any country, the species of 
these genera have more than the average number of varie- 
ties. In large genera the species are apt to be closely, but 
unequally, allied together, forming little clusters round other 
species. Species very closely allied to other species appar- 
ently have restricted ranges. In all these respects the spe- 
cies of large genera present a strong analogy with varieties. 
And we can clearly understand these analogies, if species 
once existed as varieties, and thus originated; whereas, these 
analogies are utterly inexplicable if species are independent 

We have, also, seen that it is the most flourishing or dom- 
inant species of the larger genera within each class which on 
an average yield the greatest number of varieties ; and varie- 
ties, as we shall hereafter see, tend to become converted into 
new and distinct species. Thus the larger genera tend to 
become larger; and throughout nature the forms of life 
which are now dominant tend to become still more dominant 
by leaving many modified and dominant descendants. But 
by steps hereafter to be explained, the larger genera also 
tend to break up into smaller genera. And thus, the forms 
of life throughout the universe become divided into groups 
subordinate to groups. 

Struggle for Existence 

Its bearing on natural selection — The term used in a wide sense — 
Geometrical ratio of increase — Rapid increase of naturalized 
animals and plants — Nature of the checks to increase — Competi- 
tion universal — Effects of climate — Protection from the number 
of individuals — Complex relations of all animals and plants 
throughout nature — Struggle for life most severe between indi- 
viduals and varieties of the same species : often severe between 
species of the same genus — The relation of organism to organism 
the most important of all relations. 


EFORE entering on the subject of this chapter, I must 
make a few preliminary remarks, to show how the 
struggle for existence bears on Natural Selection. It 
has been seen in the last chapter that amongst organic beings 
in a state of nature there is some individual variability: in- 
deed I am not aware that this has ever been disputed. It is 
immaterial for us whether a multitude of doubtful forms be 
called species or sub-species or varieties; what rank, for in- 
stance, the two or three hundred doubtful forms of British 
plants are entitled to hold, if the existence of any well-marked 
varieties be admitted. But the mere existence of individual 
variability and of some few well-marked varieties, though 
necessary as the foundation for the work, helps us but little 
in understanding how species arise in nature. How have all 
those exquisite adaptations of one part of the organisation 
to another part, and to the conditions of life, and of one 
organic being to another being, been perfected? We see 
these beautiful co-adaptations most plainly in the wood- 
pecker and the mistletoe; and only a little less plainly 
in the humblest parasite which clings to the hairs of a quad- 
ruped or feathers of a bird: in the structure of the beetle 
which dives through the water : in the plumed seed which is 
wafted by the gentlest breeze; in short, we see beautiful 



adaptations everywhere and in every part of the organic 

Again, it may be asked, how is it that varieties, which I 
have called incipient species, become ultimately converted 
into good and distinct species, which in most cases obviously 
differ from each other far more than do the varieties of the 
same species? How do those groups of species, which con- 
stitute what are called distinct genera, and which differ 
from each other more than do the species of the same genus, 
arise? All these results, as we shall more fully see in the 
next chapter, follow from the struggle for life. Owing to 
this struggle, variations, however slight and from whatever 
cause proceeding, if they be in any degree profitable to the 
individuals of a species, in their infinitely complex relations 
to other organic beings and to their physical conditions of 
life, will tend to the preservation of such individuals, and 
will generally be inherited by the offspring. The offspring, 
also, will thus have a better chance of surviving, for, of the 
many individuals of any species which are periodically born, 
but a small number can survive. I have called this principle, 
by which each slight variation, if viseful, is preserved, by the 
term Natural Selection, in order to mark its relation to 
man's power of selection. But the expression often used by 
Mr. Herbert Spencer of the Survival of the Fittest is more 
accurate, and is sometimes equally convenient. We have 
seen that man by selection can certainly produce great re- 
sults, and can adapt organic beings to his own uses, through 
the accumulation of slight but useful variations, given to 
him by the hand of Nature. But Natural Selection, as we 
shall hereafter see, is a power incessantly ready for action, 
and is as immeasurably superior to man's feeble efforts, as 
the works of Nature are to those of Art. 

We vv^ill now discuss in a little more detail the struggle for 
existence. In my future work this subject will be treated, 
as it well deserves, at greater length. The elder De Candolle 
and Lyell have largely and philosophically shown that all 
organic beings are exposed to severe competition. In regard 
to plants, no one has treated this subject with more spirit 
and ability than W. Herbert, Dean of Manchester, evirlcntly 
the result of his great horticultural knowledge. Nothing is 


easier than to admit in words the truth of the universal 
struggle for Hfe, or more difficult — at least, I have found it 
so — than constantly to bear this conclusion in mind. Yet 
unless it be thoroughly engrained in the mind, the whole 
economy of nature, with every fact on distribution, rarity, 
abundance, extinction, and variation, will be dimly seen or 
quite misunderstood. We behold the face of nature bright 
with gladness, we often see superabundance of food; we do 
not see, or we forget, that the birds which are idly singing 
round us mostly live on insects or seeds, and are thus con- 
stantly destroying life; or we forget how largely these song- 
sters, or their eggs, or their nestlings, are destroyed by birds 
and beasts of prey; we do not always bear in mind, that, 
though food may be now superabundant, it is not so at all 
seasons of each recurring year. 


I should premise that I use this term in a large and meta- 
phorical sense including dependence of one being on another, 
and including (which is more important) not only the life 
of the individual, but success in leaving progeny. Two 
canine animals, in a time of dearth, may be truly said to 
struggle with each other which shall get food and live. But 
a plant on the edge of a desert is said to struggle for life 
against the drought, though more properly it should be said 
to be dependent on the moisture. A plant which annually 
produces a thousand seeds, of which only one of an average 
comes to maturity, may be more truly said to struggle with 
the plants of the same and other kinds which already clothe 
the ground. The mistletoe is dependent on the apple and a 
few other trees, but can only in a far-fetched sense be said 
to struggle with these trees, for, if too many of these para- 
sites grow on the same tree, it languishes and dies. But 
several seedling mistletoes, growing close together on the 
same branch, may more truly be said to struggle with each 
other. As the mistletoe is disseminated by birds, its exist- 
ence depends on them; and it may metaphorically be said to 
struggle with other fruit-bearing plants, in tempting the 


birds to devour and thus disseminate its seeds. In these sev- 
eral senses, which pass into each other, I use for conveni- 
ence sake the general term of Struggle for Existence. 


A struggle for existence inevitably follows from the high 
rate at which all organic beings tend to increase. Every 
being, which during its natural lifetime produces several 
eggs or seeds, must suffer destruction during some period of 
its life, and during some season or occasional year, other- 
wise, on the principle of geometrical increase, its numbers 
would quickly become so inordinately great that no country 
could support the product. Hence, as more individuals are 
produced than can possibly survive, there must in every case 
be a struggle for existence, either one individual with an- 
other of the same species, or with the individuals of distinct 
species, or with the physical conditions of life. It is the 
doctrine of Malthus applied with manifold force to the whole 
animal and vegetable kingdoms; for in this case there can 
be no artificial increase of food, and no prudential restraint 
from marriage. Although some species may be now increas- 
ing, more or less rapidly, in numbers, all cannot do so, for 
the world would not hold them. 

There is no exception to the rule that every organic being 
naturally increases at so high a rate, that, if not destroyed, 
the earth would soon be covered by the progeny of a single 
pair. Even slow-breeding man has doubled in twenty-five 
years, and at this rate in less than a thousand years, there 
would literally not be standing-room for his progeny. Lin- 
naeus has calculated that if an annual plant produced only 
two seeds — and there is no plant so unproductive as this — 
and their seedlings next year produced two, and so on, then 
in twenty years there would be a million plants. The ele- 
phant is reckoned the slowest breeder of all known animals, 
and I have taken some pains to estimate its probable mini- 
mum rate of natural increase ; it will be safest to assume 
that it begins breeding when thirty years old, and goes on 
breeding till ninety years old, bringing forth six young in 
the interval, and surviving till one hundred years old; if this 


be so, after a period of from 740 to 750 years there would 
be nearly nineteen million elephants alive, descended from 
the first pair. 

But we have better evidence on this subject than mere 
theoretical calculations, namely, the numerous recorded cases 
of the astonishingly rapid increase of various animals in a 
state of nature, when circumstances have been favourable to 
them during two or three following seasons. Still more 
striking is the evidence from our domestic animals of many 
kinds which have run wild in several parts of the world; 
if the statements of the rate of increase of slow-breeding 
cattle and horses in South America, and latterly in Australia, 
had not been well authenticated, they would have been in- 
credible. So it is with plants: cases could be given of intro- 
duced plants which have become common throughout whole 
islands in a period of less than ten years. Several of the 
plants, such as the cardoon and a tall thistle, which are 
now the commonest over the wide plains of La Plata, cloth- 
ing square leagues of surface almost to the exclusion of 
everv other plant, have been introduced from Europe; and 
there are plants which now range in India, as I hear from 
Dr. Falconer, from Cape Comorin to the Himalaya, which 
have been imported from America since its discover)-. In 
such cases, and endless others could be given, no one sup- 
poses, that the fertility of the animals or plants has been 
suddenly and tem.porarily increased in any sensible degree. 
The obvious explanation is that the conditions of life have 
been higlilv favourable, and that there has constantlv been 
less destruction of the old and young, and that nearly all the 
young have been enabled to breed. Their geometrical ratio 
of increase, the result of which never fails to be surprising, 
simplv explains their extraordinarily rapid increase and wide 
diffusion in their new homes. 

In a state of nature almost ever}' full-gro\sTi plant annually 
produces seed, and amongst animals there are very few 
which do not annually pair. Hence we may confidently as- 
sert, that all plants and animals are tending to increase at a 
geometrical ratio, — that all would rapidly stock ever}- station 
in which they could anyhow exist, — and that this geomet- 
rical tendencv to increase must be checked bv destruction at 


some period of life. Our familiarity with the larger domes- 
tic animals tends, I think, to mislead us : we see no great 
destruction falling on them, but we do not keep in mind that 
thousands are annually slaughtered for food, and that in a 
state of nature an equal number would have somehow to be 
disposed of. 

The only difference between organisms which annually pro- 
duce eggs or seeds by the thousand, and those which produce 
extremely few, is, that the slow-breeders would require a 
few more years to people, under favourable conditions, a 
whole district, let it be ever so large. The condor lays a 
couple of eggs and the ostrich a score, and yet in the same 
country the condor may be the more numerous of the two,- 
the Fulmar petrel lays but one egg, yet it is believed to be 
the most numerous bird in the world. One fly deposits hun- 
dreds of eggs, and another, like the hippobosca, a single 
one; but this difference does not determine how many indi- 
viduals of the two species can be supported in a district. 
A large number of eggs is of some importance to those spe- 
cies which depend on a fluctuating amount of food, for it 
allows them rapidly to increase in number. But the real im- 
portance of a large number of eggs or seeds is to make up 
for much destruction at some period of life; and this period 
in the great majority of cases is an early one. If an animal 
can in any way protect its own eggs or young, a small num- 
ber may be produced, and yet the average stock be fully kept 
up; but if many eggs or young are destroyed, many must be 
produced, or the species will become extinct. It would suf- 
fice to keep up the full number of a tree, which lived on an 
average for a thousand years, if a single seed were produced 
once in a thousand years, supposing that this seed were never 
destroyed, and could be ensured to germinate in a fitting 
place. So that, in all cases, the average number of any ani- 
mal or plant depends only indirectly on the number of its 
eggs or seeds. 

In looking at Nature, it is most necessary to keep the fore- 
going considerations always in mind — never to forget that 
every single organic being may be said to be striving to the 
utmost to increase in numbers; that each lives by a struggle 
at some period of its life; that heavy destruction inevitably 


falls either on the young or old, during each generation or 
at recurrent intervals. Lighten any check, mitigate the de- 
struction ever so little, and the number of the species will 
almost instantaneously increase to any amount. 


The causes which check the natural tendency of each spe- 
cies to increase are most obscure. Look at the most vig- 
orous species; by as much as it swarms in numbers, by so 
much will it tend to increase still further. We know not 
exactly what the checks are even in a single instance. Nor 
will this surprise any one who reflects how ignorant we are 
on this head, even in regard to mankind, although so incom- 
parably better known than any other animal. This subject 
of the checks to increase has been ably treated by several 
authors, and I hope in a future work to discuss it at con- 
siderable length, more especially in regard to the feral ani- 
mals of South America. Here I will make only a few re- 
marks, just to recall to the reader's mind some of the chief 
points. Eggs or very young animals seem generally to suffer 
most, but this is not invariably the case. With plants there 
is a vast destruction of seeds, but, from some observations 
which I have made, it appears that the seedings suffer most 
from germinating in ground already thickly stocked with 
other plants. Seedlings, also, are destroyed in vast numbers 
by various enemies ; for instance, on a piece of ground three 
feet long and two wide, dug and cleared, and where there 
could be no choking from other plants, I marked all the 
seedlings of our native weeds as they came up, and out of 
357 no less than 295 were destroyed, chiefly by slugs and in- 
sects. If turf which has long been mown, and the case would 
be the same with turf closely browsed by quadrupeds, be let 
to grow, the more vigorous plants gradually kill the less 
vigorous, though fully grown plants ; thus out of twenty spe- 
cies growing on a little plot of mown turf (three feet by 
four) nine species perished, from the other species being al- 
lowed to grow up freely. 

The amount of food for each species of course gives the 
extreme limit to which each can increase; but very fre- 


quently it is not the obtaining food, but the serving as prey 
to other animals, which determines the average numbers of 
a species. Thus, there seems to be little doubt that the stock 
of partridges, grouse and hares on any large estate depends 
chiefly on the destruction of vermin. If not one head of 
game were shot during the next twenty years in England, 
and, at the same time, if no vermin were destroyed, there 
would, in all probability, be less game than at present, al- 
though hundreds of thousands of game animals are now 
annually shot. On the other hand, in some cases, as with 
the elephant, none are destroyed by beasts of prey; for even 
the tiger in India most rarely dares to attack a j-oung ele- 
phant protected by its dam. 

Climate plays an important part in determining the aver- 
age numbers of a species, and periodical seasons of extreme 
cold or drought seem to be the most effective of all checks. 
I estimated (chiefly from the greatly reduced numbers of 
nests in the spring) that the winter of 1854-5 destroyed four- 
fifths of the birds in my own grounds ; and this is a tremen- 
dous destruction, when we remember that ten per cent, is 
an extraordinarily severe mortality from epidemics with 
man. The action of climate seems at first sight to be quite 
independent of the struggle for existence ; but in so far as 
climate chiefly acts in reducing food, it brings on the most 
severe struggle between the individuals, whether of the same 
or of distinct species, which subsist on the same kind of 
food. Even when climate, for instance extreme cold, acts 
directly, it will be the least vigorous individuals, or those 
which have got least food through the advancing winter, 
which will suffer most. When we travel from south to 
north, or from a damp region to a dry, we invariably see 
some species gradually getting rarer and rarer, and finally 
disappearing; and the change of climate being conspicuous, 
we are tempted to attribute the whole effect to its direct 
action. But this is a false view; we forget that each species, 
even where it most abounds, is constantly suft"ering enormous 
destruction at some period of its life, from enemies or from 
competitors for the same place and food; and if these ene- 
mies or competitors be in the least degree favoured by any 
slight change of climate, they will increase in numbers; and 


as each area is already fully stocked with inhabitants, the 
other species must decrease. When we travel southward 
and see a species decreasing in numbers, we may feel sure 
that the cause lies quite as much in other species being fa- 
voured, as in this one being hurt. So it is when we travel 
northward, but in a somewhat lesser degree, for the number 
of species of all kinds, and therefore of competitors, de- 
creases northwards; hence in going northwards, or in as- 
cending a mountain, we far oftener meet with stunted forms, 
due to the directly injurious action of climate, than we do in 
proceeding southwards or in descending a mountain. When 
v/e reach the Arctic regions, or snow-capped summits, or 
absolute deserts, the struggle for life is almost exclusively 
with the elements. 

That climate acts in main part indirectly by favouring 
other species, we clearly see in the prodigious number of 
plants which in our gardens can perfectly well endure our 
climate, but which never became naturalised, for they can- 
not compete with our native plants nor resist destruction 
by our native animals. 

When a species, owing to highly favoured circumstances, 
increases inordinately in numbers in a small tract, epidemics 
— at least, this seems generally to occur with our game ani- 
mals — often ensue; and here we have a limiting check inde- 
pendent of the struggle for life. But even some of these 
so-called epidemics appear to be due to parasitic worms, 
which have from some cause, possibly in part through fa- 
cility of diffusion amongst the crowded animals, been dis- 
proportionally favoured : and here comes in a sort of struggle 
between the parasite and its prey. 

On the other hand, in many cases, a large stock of indi- 
viduals of the same species, relatively to the numbers of its 
enemies, is absolutely necessary for its preservation. Thus 
we can easily raise plenty of corn and rape-seed, &c., in our 
fields, because the seeds are in great excess, compared with 
the number of birds which feed on them ; nor can the birds, 
though having a superabundance of food at this one sea- 
son, increase in number proportionally to the supply of 
seed, as their numbers are checked during winter ; but any 
one who has tried, knows how troublesome it is to get seed 


from a few wheat or other such plants in a garden : I have 
in this case lost every single seed. This view of the neces- 
sity of a large stock of the same species for its preservation, 
explains, I believe, some singular facts in nature such as that 
of very rare plants being sometimes extremely abundant, in 
the few spots where they do exist; and that of some social 
plants being social, that is abounding in individuals, even on 
the extreme verge of their range. For in such cases, we 
may believe, that a plant could exist only where the condi- 
tions of its life were so favourable that many could exist 
together, and thus save the species from utter destruction. 
I should add that the good effects of intercrossing, and the 
ill effects of close interbreeding, no doubt come into play 
in many of these cases; but I will not here enlarge on this 




Many cases are on record showing how complex and un- 
expected are the checks and relations between organic 
beings, which have to struggle together in the same coun- 
try. I will give only a single instance, which, though a 
simple one, interested me. In Staffordshire, on the estate 
of a relation, where I had ample means of investigation, 
there was a large and extremely barren heath, which had 
never been touched by the hand of man ; but several acres 
of exactly the same nature had been enclosed twenty-five 
years previously and planted with Scotch fir. The change 
in the native vegetation of the planted part of the heath 
was most remarkable, more than is generally seen in pass- 
ing from one quite different soil to another: not only the 
proportional numbers of the heath-plants were wholly 
changed, but twelve species of plants (not counting grasses 
and carices) flourished in the plantations, which could not 
be found on the heath. The effect on the insects must have 
been still greater, for six insectivorous birds were very com- 
mon in the plantations, which were not to be seen on the 
heath; and the heath was frequented by two or three dis- 
tinct insectivorous birds. Here we see how potent has been 


the effect of the introduction of a single tree, nothing what- 
ever else having been done, with the exception of the land 
having been enclosed, so that cattle could not enter. 
But how important an element enclosure is, I plainly saw 
near Farnham, in Surrey. Here there are extensive heaths, 
with a few clumps of old Scotch firs on the distant hill- 
tops: within the last ten years large spaces have been en- 
closed, and self-sown firs are now springing up in multitudes, 
so close together that all cannot live. When I ascertained 
that these young trees had not been sown or planted, I was 
so much surprised at their numbers that I went to several 
points of view, whence I could examine hundreds of acres 
of the unenclosed heath, and literally I could not see a 
single Scotch fir, except the old planted clumps. But on 
looking closely between the stems of the heath, I found a 
multitude of seedlings and little trees which had been per- 
petually browsed down by the cattle. In one square yard, 
at a point some hundred yards distant from one of the old 
clumps, I counted thirty-two little trees; and one of them, 
with twenty-six rings of growth, had, during many years, 
tried to raise its head above the stems of the heath, and 
had failed. No wonder that, as soon as the land was en- 
closed, it became thickly clothed with vigorously growing 
young firs. Yet the heath was so extremely barren and so 
extensive that no one would ever have imagined that cattle 
would have so closely and efifectually searched it for food. 
Here we see that cattle absolutely determine the existence 
of the Scotch fir; but in several parts of the world insects 
determine the existence of cattle. Perhaps Paraguay offers 
the most curious instance of this; for here neither cattle 
nor horses nor dogs have ever run wild, though they swarm 
southward and northward in a feral state; and Azara and 
Rengger have shown that this is caused by the greater num- 
ber in Paraguay of a certain fly, which lays its eggs in the 
navels of these animals when first born. The increase of 
these flies, numerous as they are, must be habitually checked 
by some means, probably by other parasitic insects. Hence, 
if certain insectivorous birds were to decrease in Paraguay, 
the parasitic insects would probably increase; and this 
would lessen the number of the navel-frequenting flies — 


then cattle and horses would become feral, and this would 
certainly greatly alter (as indeed I have observeu in parts 
of South America) the vegetation: this again would largely 
affect the insects; and this, as we have just seen in Stafford- 
shire, the insectivorous birds, and so onwards in ever-in- 
creasing circles of complexity. Not that under nature the 
relations will ever be as simple as this. Battle within battle 
must be continually recurring with varying success; and yet 
in the long-run the forces are so nicely balanced, that the 
face of nature remains for long periods of time uniform, 
though assuredly the merest trifle would give the victory to 
one organic being over another. Nevertheless, so profound 
is our ignorance, and so high our presumption, that we 
marvel when we hear of the extinction of an organic being; 
and as we do not see the cause, we invoke cataclysms to 
desolate the world, or invent laws on the duration of the 
forms of life ! 

I am tempted to give one more instance showing how plants 
and animals, remote in the scale of nature, are bound together 
by a web of complex relations. I shall hereafter have occasion 
to show that the exotic Lobelia fulgens is never visited in 
my garden by insects, and consequently, from its peculiar 
structure, never sets a seed. Nearly all our orchidaceous 
plants absolutely require the visits of insects to remove their 
pollen-masses and thus to fertilise them. I find from experi- 
ments that humble-bees are almost indispensable to the fer- 
tilisation of the heartsease (Viola tricolor), for other bees 
do not visit this flower. I have also found that the visits of 
bees are necessary for the fertilisation of some kinds of 
clover; for instance, 20 heads of Dutch clover (Trifolium 
repens) yielded 2,290 seeds, but 20 other heads protected 
from bees produced not one. Again, 100 heads of red 
clover (T. pratense) produced 2,700 seeds, but the same 
number of protected heads produced not a single seed. 
Humble-bees alone visit red clover, as other bees cannot 
reach the nectar. It has been suggested that moths may 
fertilise the clovers; but I doubt whether they could do so 
in the case of the red clover, from their weight not being 
suflkient to depress the wing petals. Hence we may infer 
as highly probable that, if the whole genus of humble-bees 


became extinct or very rare in England, the heartsease and 
red clover would become very rare, or wholly disappear. 
The number of humble-bees in any district depends in a 
great measure upon the number of field-mice, which destroy 
their combs and nests ; and Col. Newman, who has long 
attended to the habits of humble-bees, believes that "more 
than two-thirds of them are thus destroyed all over Eng- 
land." Now the number of mice is largely dependent, as 
every one knows, on the number of cats; and Col. Newman 
says, "Near villages and small towns I have found the nests 
of humble bees more numerous than elsewhere, which I 
attribute to the number of cats that destroy the mice." 
Hence it is quite credible that the presence of a feline ani- 
mal in larje numbers in a district might determine, through 
the intervention first of mice and then of bees, the fre- 
quency of certain flowers in that district ! 

In the case of every species, many different checks, acting 
at different periods of life, and during different seasons or 
years, probably come into play ; some one check or some few 
being generally the most potent; but all will concur in deter- 
mining the average number or even the existence of the 
species. In some cases it can be shown that widely-different 
checks act on the same species in different districts. When 
we look at the plants and bushes clothing an entangled bank, 
we are tempted to attribute their proportional numbers and 
kinds to what we call chance. But how false a view is this ! 
Every one has heard that when an American forest is cut 
down, a very different vegetation springs up; but it has been 
observed that ancient Indian ruins in the Southern United 
States, which must formerly have been cleared of trees, 
now display the same beautiful diversity and proportion of 
kinds as in the surrounding virgin forest. What a struggle 
must have gone on during long centuries between the sev- 
' eral kinds of trees, each annually scattering its seeds by the 
thousand; what war between insect and insect — between 
insects, snails, and other animals with birds and beasts of 
prey — all striving to increase, all feeding on each other, or 
on the trees, their seeds and seedlings, or on the other plants 
which first clothed the ground and thus checked the growth 
of the trees ! Throw up a handful of feathers, and all fall 


to the ground according to definite laws; but how simple is 
tlie problem where each shall fall compared to that of the 
action and reaction of the innumerable plants and animals 
which have determined, in the course of centuries, the pro- 
portional numbers and kinds of trees now growing on the 
old Indian ruins ! 

The dependency of one organic being on another, as of a 
parasite on its prey, lies generally between beings remote 
in the scale of nature. This is likewise sometimes the case 
with those which may be strictly said to struggle with each 
other for existence, as in the case of locusts and grass- 
feeding quadrupeds. But the struggle will almost invariably 
be most severe between the individuals of the same species, 
for they frequent the same districts, require the same food, 
and are exposed to the same dangers. In the case of varie- 
ties of the same species, the struggle will generally be almost 
equally severe, and we sometimes see the contest soon de- 
cided : for instance, if several varieties of wheat be sown 
together, and the mixed seed be resown, some of the varie- 
ties which best suit the soil or climate, or are naturally the 
most fertile, will beat the others and so yield more seed, 
and will consequently in a few years supplant the other 
varieties. To keep up a mixed stock of even such extremely 
close varieties as the variously-coloured sweet peas, they 
must be each year harvested separately, and the seed then 
mixed in due proportion, otherwise the weaker kinds will 
steadily decrease in number and disappear. So again with 
the varieties of sheep; it has been asserted that certain 
mountain-varieties will starve out other mountain-varieties, 
so that they cannot be kept together. The same result has 
followed from keeping together different varieties of the 
medicinal leech. It may even be doubted whether the varie- 
ties of any of our domestic plants or animals have so ex- 
actly the same strength, habits, and constitution, that the 
original proportions of a mixed stock (crossing being pre- 
vented) could be kept up for half-a-dozen generations, if 
they were allowed to struggle together, in the same manner 
as beings in a state of nature, and if the seed or young were 
not annually preserved in due proportion. 



As the species of the same genus usually have, though by- 
no means invariably, much similarity in habits and consti- 
tution, and always in structure, the struggle will generally 
be more severe between them, if they come into competition 
with each other, than between the species of distinct genera. 
We see this in the recent extension over parts of the United 
States of one species of swallow having caused the decrease 
of another species. The recent increase of the missel-thrush 
in parts of Scotland has caused the decrease of the song- 
thrush. How frequently we hear of one species of rat taking 
the place of another species under the most different cli- 
mates ! In Russia the small Asiatic cockroach has every- 
where driven before it its great congener. In Australia 
the imported hive-bee is rapidly exterminating the small, 
stingless native bee. One species of charlock has been 
known to supplant another species; and so in other cases. 
We can dimly see why the competition should be most severe 
between allied forms, which fill nearly the same place in the 
economy of nature ; but probably in no one case could we 
precisely say why one species has been victorious over 
another in the great battle of life. 

A corollary of the highest importance may be deduced 
from the foregoing remarks, namely, that the structure of 
every organic being is related, in the mose essential yet often 
hidden manner, to that of all the other organic beings, with 
M'^hich it comes into competition for food or residence, or 
from which it has to escape, or on which it preys. This is 
obvious in the structure of the teeth and talons of the tiger ; . 
and in that of the legs and claws of the parasite which clings 
to the hair on the tiger's body. But in the beautifully plumed 
seed of the dandelion, and in the flattened and fringed legs 
of the water-beetle, the relation seems at first confined to 
the elements of air and water. Yet the advantage of plumed 
seeds no doubt stands in the closest relation to the land being 
already thickly clothed with other plants ; so that the seeds 
may be widely distributed and fall on unoccupied ground. 
In the water-beetle, the structure of its legs, so well adapted 


for diving, allows it to compete with other aquatic insects, 
to hunt for its own prey, and to escape serving as prey to 
other animals. 

The store of nutriment laid up within the seeds of many 
plants seems at first sight to have no sort of relation to 
other plants. But from the strong growth of young plants 
produced from such seeds, as peas and beans, when sown in 
the midst of long grass, it may be suspected that the chief 
use of the nutriment in the seed is to favour the growth of 
the seedlings, whilst struggling with other plants growing 
all around. 

Look at a plant in the midst of its range, why does it not 
double or quadruple its numbers? We know that it can per- 
fectly well withstand a little more heat or cold, dampness or 
dryness, for elsewhere it ranges into slightly hotter or colder, 
damper or drier districts. In this case we can clearly see 
that if we wish in imagination to give the plant the power of 
increasing in number, we should have to give it some ad- 
vantage over its competitors, or over the animals which prey 
on it. On the confines of its geographical range, a change 
of constitution with respect to climate would clearly be an 
advantage to our plant ; but wc have reason to believe that 
only a few plants or animals range so far, that they are de- 
stroyed exclusively by the rigour of the climate. Not until 
we reach the extreme confines of life, in the Arctic regions 
or on the borders of an utter desert, will competition cease. 
The land may be extremely cold or dry, yet there will be 
competition between some few species, or between the indi- 
viduals of the same species, for the warmest or dampest 

Hence we can see that when a plant or animal is placed 
in a new country amongst new competitors, the conditions 
of its life will generally be changed in an essential manner, 
although the climate may be exactly the same as in its 
former home. If its average numbers are to increase in its 
new home, we should have to modify it in a different way to 
what we should have had to do in its native country; for we 
should have to give it some advantage over a different set 
of competitors or enemies. 

It is good thus to try in imagination to give to any one 


species an advantage over another. Probably in no single 
instance should we know what to do. This ought to con- 
vince us of our ignorance on the mutual relations of all 
organic beings ; a conviction as necessary, as it is difificult 
to acquire. All that we can do, is to keep steadily in mind 
that each organic being is striving to increase in a geomet- 
rical ratio; that each at some period of its life, during some 
season of the year, during each generation or at intervals, 
has to struggle for life and to suffer great destruction. 
When we reflect on this struggle, we may console ourselves 
with the full belief, that the war of nature is not inces- 
sant, that no fear is felt, that death is generally prompt, 
and that the vigorous, the healthy, and the happy survive 
and multiply. 

Natural Selection ; or the Survival of the Fittest 

Natural Selection — its power compared with man's selection — its 
power on characters of trifling importance — its power at all ages 
and on both sexes — Sexual selection — On the generality of inter- 
crosses between individuals of the same species — Circumstances 
favourable and unfavourable to the results of Natural Selection, 
namely, intercrossing, isolation, number of individuals — Slow 
action — Extinction caused by Natural Selection — Divergence of 
Character, related to the diversity of inhabitants of any small 
area, and to naturalisation — Action of Natural Selection, through 
divergence of Character and Extinction, on the descendants from 
a common parent — Explains the grouping of all organic beings — 
Advance in organisation — Low forms preserved — Convergence of 
Character — Indefinite multiplication of species — Summary. 

HOW will the struggle for existence, briefly discussed 
in the last chapter, act in regard to variation? Can 
the principle of selection, v^'hich we have seen is so 
potent in the hands of man, apply under nature? I think 
we shall see that it can act most efficiently. Let the endless 
number of slight variations and individual differences occur- 
ring in our domestic productions, and, in a lesser degree, in 
those under nature, be borne in mind ; as well as the strength 
of the hereditary tendency. Under domestication, it may be 
truly said that the whole organisation becomes in some degree 
plastic. But the variability, which we almost universally 
meet with in our domestic productions, js not directly pro- 
duced, as Hooker and Asa Gray have well remarked, by man ; 
he can neither originate varieties, nor prevent their occur- 
rence ; he can only preserve and accumulate such as do occur. 
Unintentionally he exposes organic beings to new and chang- 
ing conditions of life, and variability ensues; but similar 
changes of conditions might and do occur under nature. Let 
it also be borne in mind how infinitely complex and close- 
fitting are the mutual relations of all organic beings to each 
other and to their physical conditions of life; and conse- 



quently what infinitely varied diversities of structure might 
be of use to each being under changing conditions of life. 
Can it, then, be thought improbable, seeing that variations 
useful to man have undoubtedly occurred, that other vari- 
ations useful in some wzy to each being in the great and com- 
plex battle of life, should occur in the course of many suc- 
cessive generations? If such do occur, can we doubt (re- 
membering that many more individuals are born than can 
possibly survive) that individuals having any advantage, 
however slight, over others, would have the best chance of 
surviving and of procreating their kind ? On the other hand, 
we may feel sure that any variation in the least degree injuri- 
ous would be rigidly destroyed. This preservation of favour- 
able individual differences and variations, and the destruction 
of those which are injurious, I have called Natural Selection, 
or the Survival of the Fittest. Variations neither useful nor 
injurious would not be affected by natural selection, and 
would be left either a fluctuating element, as perhaps we see 
in certain polymorphic species, or would ultimately become 
fixed, owing to the nature of the organism and the nature of 
the conditions. 

Several writers have misapprehended or objected to the 
term Natural Selection. Some have even imagined that nat- 
ural selection induces variability, whereas it implies only the 
preservation of such variations as arise and are beneficial to 
the being under its conditions of life. No one objects to 
agriculturists speaking of the potent effects of man's selec- 
tion; and in this case the individual differences given by 
nature, which man for some object selects, must of necessity 
first occur. Others have objected that the term selection im- 
plies conscious choice in the animals which become modified; 
and it has even been urged that, as plants have no volition, 
natural selection is not applicable to them! In the literal 
sense of the word, no doubt, natural selection is a false term ; 
but who ever objected to chemists speaking of the elective 
affinities of the various elements? — and yet an acid cannot 
strictly be said to elect the base with which it in preference 
combines. It has been said that I speak of natural selection 
as an active power or Deity; but who objects to an author 
speaking of the attraction of gravity as ruling the movements 


of the planets? Every one knows what is meant and is im- 
plied by such metaphorical expressions ; and they are almost 
necessary for brevity. So again it is difficult to avoid per- 
sonifying the word Nature; but I mean by Nature, only the 
aggregate action and product of many natural laws, and by 
laws the sequence of events as ascertained by us. With a 
little familiarity such superficial objections will be forgotten. 

We shall best understand the probable course of natural 
selection by taking the case of a country undergoing some 
slight physical change, for instance, of climate. The propor- 
tional numbers of its inhabitants will almost immediately un- 
dergo a change, and some species will probably become ex- 
tinct. We may conclude, from what we have seen of the in- 
timate and complex manner in which the inhabitants of each 
country are bound together, that any change in the numerical 
proportions of the inhabitants, independently of the change 
of climate itself, would seriously affect the others. If the 
country were open on its borders, new forms would certainly 
immigrate, and this would likewise seriously disturb the rela- 
tions of some of the former inhabitants. Let it be remem- 
bered how powerful the influence of a single introduced tree 
or mammal has been shown to be. But in the case of an 
island, or of a country partly surrounded by barriers, into 
which new and better adapted forms could not freely enter, 
we should then have places in the economy of nature which 
would assuredly be better filled up, if some of the original 
inhabitants were in some manner modified; for, had the are'a 
been open to immigration, these same places would have been 
seized on by intruders. In such cases, slight modifications, 
which in any way favoured the individuals of any species, 
by better adapting them to their altered conditions, would 
tend to be preserved; and natural selection would have free 
scope for the work of improvement. 

We have good reason to believe, as shown in the first chap- 
ter, that changes in the conditions of life give a tendency to 
increased variability; and in the foregoing cases the con- 
ditions have changed, and this would manifestly be favour- 
able to natural selection, by affording a better chance of the 
occurrence of profitable variations. Unless such occur, nat- 
ural selection can do nothing. Under the term of "vari- 


ations," it must never be forgotten that mere individual dif- 
ferences are included. As man can produce a great result 
with his domestic animals and plants by adding up in any 
given direction individual differences, so could natural selec- 
tion, but far more easily from having incomparably longer 
time for action. Nor do I believe that any great physical 
change, as of climate, or any unusual degree of isolation to 
check immigration, Is necessary in order that new and un- 
occupied places should be left, for natural selection to fill up 
by improving some of the varying inhabitants. For as all 
the inhabitants of each country are struggling together with 
nicely balanced forces, extremely slight modifications in the 
structure or habits of one species would often give it an ad- 
vantage over others; and still further modifications of the 
same kind would often still further increase the advantage, 
as long as the species continued under the same conditions 
of life and profited by similar means of subsistence and de- 
fence. No country can be named in which all the native in- 
habitants are now so perfectly adapted to each other and to 
the physical conditions under which they live, that none of 
them could be still better adapted or improved; for in all- 
countries, the natives have been so far conquered by natural- 
ised productions, that they have allowed some foreigners to 
take firm possession of the land. And as foreigners have 
thus in every country beaten some of the natives, we may 
safely conclude that the natives might have been modified 
with advantage, so as to have better resisted the intruders. 

As man can produce, and certainly has produced, a great 
result by his methodical and unconscious means of selection, 
what may not natural selection effect? Man can act only on 
external and visible characters : Nature, if I may be allowed 
to personify the natural preservation or survival of the fit- 
test, cares nothing for appearances, except in so far as they 
are useful to any being. She can act on every internal organ, 
on every shade of constitutional difference, on the whole 
machinery of life. Man selects only for his own good: Na- 
ture only for that of the being which she tends. Every 
selected character is fully exercised by her, as is implied by 
the fact of their selection. Man keeps the natives of many 
climates in the same country; he seldom exercises each se- 


lected character in some peculiar and filling manner ; he feeds 
a long and a short beaked pigeon on the same food ; he does 
not exercise a long-backed or long-legged quadruped in any 
peculiar manner ; he exposes sheep with long and short wool 
to the same climate. He does not allow the most vigorous 
males to struggle for the females. He does not rigidly de- 
stroy all inferior animals, but protects during each varying 
season, as far as lies in his power, all his productions. He 
often begins his selection by some half-monstrous form ; or 
at least by some modification prominent enough to catch the 
eye or to be plainly useful to him. Under nature, the slight- 
est differences of structure or constitution may well turn the 
nicely-balanced scale in the struggle for life, and so be pre- 
served. How fleeting are the wishes and efforts of man ! 
how short his time ! and consequently how poor will be his 
results, compared with those accumulated by Nature during 
whole geological periods? Can we wonder, then, that Na- 
ture's productions should be far "truer" in character than 
man's productions ; that they should be infinitely better 
adapted to the most complex conditions of life, and should 
plainly bear the stamp of far higher workmanship ? 

It may metaphorically be said that natural selection is daily 
and hourly scrutinising, throughout the world, the slightest 
variations; rejecting those that are bad, preserving and add- 
ing up all that are good; silently and insensibly working, 
whenever and wherever opportunity offers, at the improve- 
ment of each organic being in relation to its organic and in- 
organic conditions of life. We see nothing of these slow 
changes in progress, until the hand of time has marked the 
lapse of ages, and then so imperfect is our view into long- 
past geological ages, that we see only that the forms of life 
are now different from what they formerly were. 

In order that any great amount of modification should be 
effected in a species, a variety when once formed must again, 
perhaps after a long interval of time, vary or present indi- 
vidual differences of the same favourable nature as before ; 
and these must be again preserved, and so onwards step by 
step. Seeing that individual differences of the same kind 
perpetually recur, this can hardly be considered as an unwar- 
rantable assumption. But whether it is true, we can judge 

D— lie XI 


only by seeing how far the hypothesis accords with and ex- 
plains the general phenomena of nature. On the other hand, 
the ordinary belief that the amount of possible variation is 
a strictly limited quantity is likewise a simple assumption. 

Although natural selection can act only through and for 
the good of each being, yet characters and structures, which 
we are apt to consider as of very trifling importance, may 
thus be acted on. When we see leaf-eating insects green, 
and bark- feeders mottled-grey; the alpine ptarmigan white in 
winter, the red-grouse the colour of heather, we must believe 
that these tints are of service to these birds and insects in 
preserving them from danger. Grouse, if not destroyed at 
some period of their lives, would increase in countless num- 
bers ; they are known to suffer largely from birds of prey; 
and hawks are guided by eyesight to their prey — so much so, 
that on parts of the Continent persons are warned not to keep 
white pigeons, as being the most liable to destruction. Hence 
natural selection might be effective in giving the proper 
colour to each kind of grouse, and in keeping that colour, 
when once acquired, true and constant. Nor ought we to 
think that the occasional destruction of an animal of any par- 
ticular colour would produce little effect: we should remem- 
ber how essential it is in a flock of white sheep to destroy a 
lamb with the faintest trace of black. We have seen how 
the colour of the hogs, which feed on the "paint-root" in 
Virginia, determines whether they shall live or die. In 
plants, the down on the fruit and the colour of the flesh are 
considered by botanists as characters of the most trifling im- 
portance : yet we hear from an excellent horticulturist. Down- 
ing, that in the United States smooth-skinned fruits suffer 
far more from a beetle, a Curculio, than those with down; 
that purple plums suffer far more from a certain disease than 
yellow plums, whereas another disease attacks yellow-fleshed 
peaches far more than those with other coloured flesh. If, 
with all the aids of art, these slight differences make a great 
difference in cultivating the several varieties, assuredly, in a 
state of nature, where the trees would have to struggle with 
other trees and with a host of enemies, such differences 
would effectually settle which variety, whether a smooth or 
downy, a yellow or purple fleshed fruit, should succeed. 


In looking at many small points of difference between 
species, which, as far as our ignorance permits us to judge, 
seem quite unimportant, we must not forget that climate, 
food, &c., have no doubt produced some direct effect. It is 
also necessary to bear in mind that, owing to the law of cor- 
relation, when one part varies, and the variations are accu- 
mulated through natural selection, other modifications, often 
of the most unexpected nature, will ensue. 

As we see that those variations which, under domestica- 
tion, appear at any particular period of life, tend to reappear 
in the offspring at the same period; — for instance, in the 
shape, size, and flavour of the seeds of the many varieties of 
our culinary and agricultural plants ; in the caterpillar and 
cocoon stages of the varieties of the silkworm ; in the eggs oi 
poultry, and in the colour of the down of their chickens; in 
the horns of our sheep and cattle when nearly adult; — so in 
a state of nature natural selection will be enabled to act on 
and modify organic beings at any age, by the accumulation of 
variations profitable at that age, and by their inheritance at 
a corresponding age. If it profit a plant to have its seeds 
more and more widely disseminated by the wind, I can see no 
greater difficulty in this being effected through natural selec- 
tion, than in the cotton-planter increasing and improving by 
selection the down in the pods on his cotton-trees. Natural 
selection may modify and adapt the larva of an insect to a 
score of contingencies, wholly different from those which con- 
cern the mature insect; and these modifications may effect, 
through correlation, the structure of the adult. So, con- 
versely, modifications in the adult may affect the structure 
of the larva ; but in all cases natural selection will ensure that 
they shall not be injurious: for if they were so, the species 
w'ould become extinct. 

Natural selection will modif)^ the structure of the young in 
relation to the parent, and of the parent in relation to the 
young. In social animals it will adapt the structure of each 
individual for the benefit of the whole community; if the 
community profits by the selected change. What natural 
selection cannot do, is to modify the structure of one species, 
without giving it any advantage, for the good of another 
species ; and though statements to this effect may be found in 


works of natural history, I cannot find one case which will 
bear investigation. A structure used only once in an ani- 
mal's life, if of high importance to it, might be modified to 
any extent by natural selection; for instance, the great jaws 
possessed by certain insects, used exclusively for opening the 
cocoon — or the hard tip to the beak of unhatched birds, used 
for breaking the egg. It has been asserted, that of the best 
short-beaked tumbler-pigeons a greater number perish in the 
egg than are able to get out of it; so that fanciers assist in 
the act of hatching. Now if nature had to make the beak of 
a full-grown pigeon very short for the bird's own advantage, 
the process of modification would be very slow, and there 
would be simultaneously the most rigorous selection of all the 
young birds within the egg, which had the most powerful and 
hardest beaks, for all with weak beaks would inevitably per- 
ish ; or, more delicate and more easily broken shells might be 
selected, the thickness of the shell being known to vary like 
every other structure. 

It may be well here to remark that with all beings there 
must be much fortuitous destruction, which can have little or 
no influence on the course of natural selection. For instance 
a vast n'umber of eggs or seeds are annually devoured, and 
these could be modified through natural selection only if they 
varied in some manner which protected them from their ene- 
mies. Yet many of these eggs or seeds would perhaps, if not 
destroyed, have yielded individuals better adapted to their 
conditions of life than any of those which happened to sur- 
vive. So again a vast number of mature animals and plants, 
whether or not they be the best adapted to their conditions, 
must be annually destroyed by accidental causes, which would 
not be in the least degree mitigated by certain changes of 
structure or constitution which would in other ways be bene- 
ficial to the species. But let the destruction of the adults be 
ever so heavy, if the number which can exist in any district 
be not wholly kept down by such causes, — or again let the 
destruction of eggs or seeds be so great that only a hundredth 
or a thousandth part are developed, — yet of those which do 
survive, the best adapted individuals, supposing that there is 
any variability in a favourable direction, will tend to propa- 
gate their kind in larger numbers than the less well adapted. 


If the numbers be wholly kept down by the causes just indi- 
cated, as will often have been the case, natural selection will 
be powerless in certain beneficial directions; but this is no 
valid objection to its efficiency at other times and in other 
ways; for we are far from having any reason to suppose that 
many species ever undergo modification and improvement at 
the same time in the same area. 


Inasmuch as peculiarities often appear under domestica- 
tion in one sex and become hereditarily attached to that sex, 
so no doubt it will be under nature. Thus it is rendered pos- 
sible for the two sexes to be modified through natural selec- 
tion in relation to different habits of life, as is sometimes the 
case ; or for one sex to be modified in relation to the other 
sex, as commonly occurs. This leads me to say a few words 
on what I have called Sexual Selection. This form of selec- 
tion depends, not on a struggle for existence in relation to 
other organic beings or to external conditions, but on a 
struggle between the individuals of one sex, generally the 
males, for the possession of the other sex. The result is not 
death to the unsuccessful competitor, but few or no offspring. 
Sexual selection is, therefore, less rigorous than natural se- 
lection. Generally, the most vigorous males, those which are 
best fitted for their places in nature, will leave most progeny. 
But in many cases, victory depends not so much on general 
vigour, as on having special weapons, confined to the male 
sex. A hornless stag or spurless cock would have a poor 
chance of leaving numerous offspring. Sexual selection, by 
always allowing the victor to breed, might surely give in- 
domitable courage, length to the spur, and strength to the 
wing to strike in the spurred leg, in nearly the same manner 
as does the brutal cockfighter by the careful selection of his 
best cocks. How low in the scale of nature the law of battle 
descends, I know not ; male alligators have been described as 
fighting, bellowing, and whirling round, like Indians in a 
war-dance, for the possession of the females: male salmons 
have been observed fighting all day long; male stag-beetles 
sometimes bear wounds from the huge mandibles of other 


males ; the males of certain hymenopterous insects have been 
frequently seen by that inimitable observer M. Fabre, fighting 
for a particular female who sits by, an apparently uncon- 
cerned beholder of the struggle, and then retires with the 
conqueror. The war is, perhaps, severest between the males 
of polygamous animals, and these seem oftenest provided 
with special weapons. The males of carnivorous animals 
are already well armed ; though to them and to others, special 
means of defence may be given through means of sexual 
selection, as the mane of the lion, and the hooked jaw to the 
male salmon ; for the shield may be as important for victory, 
as the sword or spear. 

Amongst birds, the contest is often of a more peaceful 
character. All those who have attended to the subject, be- 
lieve that there is the severest rivalry betwen the males of 
many species to attract, by singing, the females. The rock- 
thrush of Guiana, birds of paradise, and some others, congre- 
gate ; and successive males display with the most elaborate 
care, and show off in the best manner, their gorgeous plu- 
mage; they likewise perform strange antics before the fe- 
males, which, standing by as spectators, at last choose the most 
attractive partner. Those who have closely attended to birds 
in confinement well know that they often take individual 
preferences and dislikes; thus Sir R. Heron has described 
how a pied peacock was eminently attractive to all his hen 
birds. I cannot here enter on the necessary details; but if 
man can in a short time give beauty and an elegant carriage 
to his bantams, according to his standard of beauty, I can see 
no good reason to doubt that female birds, by selecting, dur- 
ing thousands of generations, the most melodious or beautiful 
males, according to their standard of beauty, might produce a 
marked effect. Some well-known laws, with respect to the 
plumage of male and female birds, in comparison with the 
plumage of the young, can partly be explained through the 
action of sexual selection on variations occurring at different 
ages, and transmitted to the males alone or to both sexes at 
corresponding ages; but I have not space here to enter on 
this subject. 

Thus it is, as I believe, that when the males and females of 
any animal have the same general habits of life, but differ in 


structure, colour, or ornament, such differences have been 
mainly caused by sexual selection : that is, by individual males 
having had, in successive generations, some slight advantage 
over other males, in their weapons, means of defence, or 
charms, which they have transmitted to their male offspring 
alone. Yet, I would not wish to attribute all sexual differ- 
ences to this agency: for we see in our domestic animals 
peculiarities arising and becoming attached to the male sex, 
which apparently have not been augmented through selection 
by man. The tuft of hair on the breast of the wild turkey- 
cock cannot be of any use, and it is doubtful whether it can 
be ornamental in the eyes of the female bird ; — indeed, had 
the tuft appeared under domestication, it would have been 
called a monstrosity. 


In order to make it clear how, as I believe, natural selec- 
tion acts, I must beg permission to give one or two imaginary 
illustrations. Let us take the case of a wolf, which preys on 
various animals, securing some by craft, some by strength, 
and some by fleetness ; and let us suppose that the fleetest 
prey, a deer for instance, had from any change in the country 
increased in numbers, or that other prey had decreased in 
numbers, during that season of the year when the wolf was 
hardest pressed for food. Under such circumstances the 
swiftest and slimmest wolves would have the best chance of 
surviving and so be preserved or selected, — provided always 
that they retained strength to master their prey at this or 
some other period of the year, when they were compelled to 
prey on other animals. I can see no more reason to doubt 
that this would be the result, than that man should be able to 
improve the fleetness of his greyhounds by careful and 
methodical selection, or by that kind of unconscious selection 
which follows from each man trying to keep the best dogs 
without any thought of modifying the breed. I may add, 
that, according to Mr. Pierce, there are two varieties of the 
wolf inhabiting the Catskill Mountains, in the United States, 
one with a light greyhound-like form, which pursues deer, 


and the other more bulky, with shorter legs, which more fre- 
quently attacks the shepherd's flocks. 

It should be observed that, in the above illustration, I speak 
of the slimmest individual wolves, and not of any single 
strongly-marked variation having been preserved. In former 
editions of this work I sometimes spoke as if this latter alter- 
native had frequently occurred. I saw the great importance 
of individual differences, and this led me fully to discuss the 
results of unconscious selection by man, which depends on 
the preservation of all the more or less valuable individuals, 
and on the destruction of the worst. I saw, also, that the 
preservation in a state of nature of any occasional deviation 
of structure, such as a monstrosity, would be a rare event; 
and that, if at first preserved, it would generally be lost by 
subsequent intercrossing with ordinary individuals. Never- 
theless, until reading an able and valuable article in the 
'North British Review' (1867), I did not appreciate how 
rarely single variations, whether slight or strongly-marked, 
could be perpetuated. The author takes the case of a pair 
of animals, producing during their lifetime two hundred off- 
spring, of which, from various causes of destruction, only two 
on an average survive to pro-create their kind. This is 
rather an extreme estimate for most of the higher animals, 
but by no means so for many of the lower organisms. He 
then shows that if a single individual were born, which varied 
in some manner, giving it twice as good a chance of life as 
that of the other individuals, yet the chances would be 
strongly against its survival. Supposing it to survive and to 
breed, and that half its young inherited the favourable vari- 
ation ; still, as the Reviewer goes on to show, the young would 
have only a slightly better chance of surviving and breeding; 
and this chance would go on decreasing in the succeeding 
generations. The justice of these remarks cannot, I think, 
be disputed. If, for instance, a bird of some kind could pro- 
cure its food more easily by having its beak curved, and if 
one were born with its beak strongly curved, and which con- 
sequently flourished, nevertheless there would be a very poor 
chance of this one individual perpetuating its kind to the ex- 
clusion of the common form ; but there can hardly be a doubt, 
judging by what we see taking place under domestication, 


that this result would follow from the preservation during 
many generations of a large number of individuals with more 
or less strongly curved beaks, and from the destruction of a 
still larger number with the straightest beaks. 

It should not, however, be overlooked that certain rather 
strongly marked variations, which no one would rank as mere 
individual differences, frequently recur owing to a similar 
organisation being similarly acted on — of which fact numer- 
ous instances could be given with our domestic productions. 
In such cases, if the varying individual did not actually trans- 
mit to its offspring its newly-acquired character, it would un- 
doubtedly transmit to them, as long as the existing conditions 
remained the same, a still stronger tendency to vary in the 
same manner. There can also be little doubt that the ten- 
dency to vary in the same manner has often been so strong 
that all the individuals of the same species have been simi- 
larly modified without the aid of any form of selection. Or 
only a third, fifth, or tenth part of the individuals may have 
been thus affected, of which fact several instances could be 
given. Thus Graba estimates that about one-fifth of the 
guillemots in the Faroe Islands consist of a variety so well 
marked, that it was formerly ranked as a distinct species 
under the name of Uria lacrymans. In cases of this kind, if 
the variation were of a beneficial nature, the original form 
would soon be supplanted by the modified form, through the 
survival of the fittest. 

To the effects of intercrossing in eliminating variations of all 
kinds, I shall have to recur ; but it may be here remarked that 
most animals and plants keep to their proper homes, and do 
not needlessly wander about ; we see this even with migratory 
birds, which almost always return to the same spot. Conse- 
quently each newly-formed variety would generally be at 
first local, as seems to be the common rule with varieties in a 
state of nature; so that similarly modified individuals would 
soon exist in a small body together, and would often breed 
together. If the new variety were successful in its battle for 
life, it would slowly spread from a central district, competing 
with and conquering the unchanged individuals on the mar- 
gins of an ever-increasing circle. 

It may be worth while to give another and more complex 


illustration of the action of natural selection. Certain plants 
excrete sweet juice, apparently for the sake of eliminating 
something injurious from the sap: this is effected, for in- 
stance, by glands af the base of the stipules in some Legu- 
minosse, and at the backs of the leaves of the common laurel. 
This juice, though small in quantity, is greedily sought by 
insects ; but their visits do not in any way benefit the plant. 
Now, let us suppose that the juice or nectar was excreted 
from the inside of the flowers of a certain number of plants 
of any species. Insects in seeking the nectar would get 
dusted with pollen, and would often transport it from one 
flower to another. The flowers of two distinct individuals 
of the same species would thus get crossed; and the act of 
crossing, as can be fully proved, gives rise to vigorous seed- 
lings, which consequently would have the best chance of flour- 
ishing and surviving. The plants which produced flowers 
with the largest glands or nectaries, excreting much nectar, 
would oftenest be visited by insects, and would oftenest be 
crossed; and so in the long-run would gain the upper hand 
and form a local variety. The flowers, also, which had their 
stamens and pistils placed, in relation to the size and habits 
of the particular insect which visited them, so as to favour 
in any degree the transportal of the pollen, would likewise 
be favoured. We might have taken the case of insects visit- 
ing flowers for the sake of collecting pollen instead of nectar; 
and as pollen is formed for the sole purpose of fertilisation, 
its destruction appears to be a simple loss to the plant; yet if 
a little pollen were carried, at first occasionally and^then 
habitually, by the pollen-devouring insects from flower to 
flower, and a cross thus effected, although nine-tenths of the 
pollen were destroyed, it might still be a great gain to the 
plant to be thus robbed ; and the individuals which produced 
more and more pollen, and had larger anthers, would be 

When our plant, by the above process long continued, had 
been rendered highly attractive to insects, they would, unin- 
tentionally on their part, regularly carry pollen from flower 
to flower; and that they do this effectually, I could easily 
show by many striking facts. I will give only one, as like- 
wise illustrating one step in the separation of the sexes of 


plants. Some holly-trees bear only male flowers, which have 
four stamens producing a rather small quantity of pollen, and 
a rudimentary pistil ; other holly-trees bear only female 
flowers ; these have a full-sized pistil, and four stamens with 
shrivelled anthers, in which not a grain of pollen can be de- 
tected. Having found a female tree exactly sixty yards from 
a male tree, I put the stigmas of twenty flowers, taken from 
different branches, under the microscope, and on all, without 
exception, there were a few pollen-grains, and on some a 
profusion. As the wind had set for several days from the 
female to the male tree, the pollen could not thus have been 
carried. The weather had been cold and boisterous, and 
therefore not favourable to bees, nevertheless every female 
flower which I examined had been effectually fertilised by the 
bees, which had flown from tree to tree in search of nectar. 
But to return to our imaginary case : as soon as the plant had 
been rendered so highly attractive to insects that pollen was 
regularly carried from flower to flower, another process 
might commence. No naturalist doubts the advantage of 
what has been called the "physiological division of labour;" 
hence we may believe that it would be advantageous to a 
plant to produce stamens alone in one flower or on one whole 
plant, and pistils alone in another flower or on another plant. 
In plants under culture and placed under new conditions of 
life, sometimes the male organs and sometimes the female 
organs become more or less impotent; now if we suppose this 
to occur in ever so slight a degree under nature, then, as 
pollen is already carried regularly from flower to flower, and 
as a more complete separation of the sexes of our plant would 
be advantageous on the principle of the division of labour, 
individuals with this tendency more and more increased, would 
be continually favoured or selected, until at last a complete 
separation of the sexes might be effected. It would take up 
too much space to show the various steps, through dimorph- 
ism and other means, by which the separation of the sexes in 
plants of various kinds is apparently now in progress; but I 
may add that some of the species of holly in North America, 
are, according to Asa Gray, in an exactly intermediate con- 
dition, or, as he expresses it, are more or less dicEciously 


Let us now turn to the nectar-feeding insects; we may 
suppose the plant, of which we have been slowly increasing 
the nectar by continued selection, to be a common plant ; and 
that certain insects depended in main part on its nectar for 
food. I could give many facts showing how anxious bees are 
to save time: for instance, their habit of cutting holes and 
sucking the nectar at the bases of certain flowers, which with 
a very little more trouble, they can enter by the mouth. 
Bearing such facts in mind, it may be believed that under cer- 
tain circumstances individual differences in the curvature or 
length of the proboscis, &c., too slight to be appreciated by 
us, might profit a bee or other insect, so that certain indi- 
viduals would be able to obtain their food more quickly than 
others; and thus the communities to which they belonged 
would flourish and throw off many swarms inheriting the 
same peculiarities. The tubes of the corolla of the common 
red and incarnate clovers (Trifolium pratense and incar- 
natum) do not on a hasty glance appear to differ in length; 
yet the hive-bee can easily suck the nectar out of the incar- 
nate clover, but not out of the common red clover, which is 
visited by humble-bees alone; so that whole fields of the red 
clover offer in vain an abundant supply of precious nectar to 
the hive-bee. That this nectar is much liked by the hive-bee is 
certain ; for I have repeatedly seen, but only in the autumn, 
many hive-bees sucking the flowers through holes bitten in 
the base of the tube by humble-bees. The difference in the 
length of the corolla in the two kinds of clover, which deter- 
mines the visits of the hive-bee, must be very trifling; for I 
have been assured that when red clover has been mown, the 
flowers of the second crop are somewhat smaller, and that 
these are visited by many hive-bees. I do not know whether 
this statement is accurate; nor whether another published 
statement can be trusted, namely, that the Ligurian bee, which 
is generally considered a mere variety of the common hive- 
bee, and which freely crosses with it, is able to reach and suck 
the nectar of the red clover. Thus, in a country where this kind 
of clover abounded, it might be a great advantage to the 
hive-bee to have a slightly longer or differently constructed 
proboscis. On the other hand, as the fertility of this clover 
absolutely depends on bees visiting the flowers, if humble- 


bees were to become rare in any country, it might be a great 
advantage to the plant to have a shorter or more deeply di- 
vided corolla, so that the hive-bees should be enabled to suck 
its flowers. Thus I can understand how a flower and a bee 
might slowly become, either simultaneously or one after the 
other, modified and adapted to each other in the most perfect 
manner, by the continued preservation of all the individuals 
which presented slight deviations of structure mutually fa- 
vourable to each other. 

I am well aware that this doctrine of natural selection, 
exemplified in the above imaginary instances, is open to the 
same objections which were first urged against Sir Charles 
Lyell's noble views on "the modern changes of the earth, as 
illustrative of geology;" but we now seldom hear the agencies 
which we see still at work, spoken of as trifling or insignifi- 
cant, when used in explaining the excavation of the deepest 
valleys or the formation of long lines of inland cliffs. Nat- 
ural selection acts only by the preservation and accumulation 
of small inherited modifications, each profitable to the pre- 
served being; and as modern geology has almost banished 
such views as the excavation of a great valley by a single 
diluvial wave, so will natural selection banish the belief of 
the continued creation of new organic beings, or of any great 
and sudden modification in their structure. 


I must here introduce a short digression. In the case of 
animals and plants with separated sexes, it is of course obvi- 
ous that two individuals must always (with the exception of 
the curious and not well understood cases of parthenogene- 
sis) unite for each birth; but in the case of hermaphrodites 
this is far from obvious. Nevertheless there is reason to be- 
lieve that with all hermaphrodites two individuals, either 
occasionally or habitually, concur for the reproduction of 
their kind. This view was long ago doubtfully suggested by 
Sprengel, Knight and Kolreutcr. We shall presently sec its 
importance ; but I must here treat the subject with extreme 
brevity, though I have the materials prepared for an ample 
discussion. All vertebrate animals, all insects, and some 


other large groups of animals, pair for each birth. Modern 
research has much diminished the number of supposed her- 
maphrodites, and of real hermaphrodites a large number 
pair; that is, two individuals regularly unite for reproduc- 
tion, which is all that concerns us. But still there are many- 
hermaphrodite animals which certainly do not habitually pair, 
and a vast majority of plants are hermaphrodites. What 
reason, it may be asked, is there for supposing in these cases 
that two individuals ever concur in reproduction? As it is 
impossible here to enter on details, I must trust to some gen- 
eral considerations alone. 

In the first place, I have collected so large a body of facts, 
and made so many experiments, showing, in accordance with 
the almost universal belief of breeders, that with animals and 
plants a cross between different varieties, or between indi- 
viduals of the same variety but of another strain, gives vigour 
and fertility to the offspring; and on the other hand, that 
close interbreeding diminishes vigour and fertility ; that these 
facts alone incline me to believe that it is a general law of 
nature that no organic being fertilises itself for a perpetuity 
of generations; but that a cross with another individual is 
occasionally — perhaps at long intervals of time — indispen- 

On the belief that this is a law of nature, we can, I think, 
understand several large classes of facts, such as the follow- 
ing, which on any other view are inexplicable. Every 
hybridizer knows how unfavourable exposure to wet is to 
the fertilisation of a flower, yet what a multitude of flowers 
have their anthers and stigmas fully exposed to the weather ! 
If an occasional cross be indispensable, notwithstanding that 
the plant's own anthers and pistil stand so near each other 
as almost to insure self-fertilisation, the fullest freedom for 
the entrance of pollen from another individual will explain 
the above state of exposure of the organs. Many flowers, on 
the other hand, have their organs of fructification closely en- 
closed, as in the great papilionaceous or pea- family; but these 
almost invariably present beautiful and curious adaptations 
in relation to the visits of insects. So necessary are the visits 
of bees to many papilionaceous flowers, that their fertility is 
greatly diminished if these visits be prevented. Now, it is 


scarcely possible for insects to fly from flower to flower, and 
not to carry pollen from one to the other, to the great good 
of the plant. Insects act like a camel-hair pencil, and it is 
sufficient, to ensure fertilisation, just to touch with the same 
brush the anthers of one flower and then the stigma of an- 
other; but it must not be supposed that bees would thus pro- 
duce a multitude of hybrids between distinct species; for if a 
plant's own pollen and that from another species are placed 
on the same stigma, the former is so prepotent that it in- 
variably and completely destroys, as has been shown by Gart- 
ner, the influence of the foreign pollen. 

When the stamens of a flower suddenly spring towards the 
pistil, or slowly move one after the other towards it, the con- 
trivance seems adapted solely to ensure self-fertilisation ; and 
no doubt it is useful for this end : but the agency of insects is 
often required to cause the stamens to spring forward, as 
Kolreuter has shown to be the case with the barberry ; and in 
this very genus, which seems to have a special contrivance 
for self-fertilisation, it is well known that, if closely-allied 
forms or varieties are planted near each other, it is hardly 
possible to raise pure seedlings, so largely do they naturally 
cross. In numerous other cases, far from self-fertilisation 
being favoured, there are special contrivances which eflfec- 
tually prevent the stigma receiving pollen from its own 
flower, as I could show from the works of Sprengel and 
others, as well as from my own observations : for instance, 
in Lobelia fulgens, there is a really beautiful and elaborate 
contrivance by which all the infinitely numerous pollen- 
granules are swept out of the conjoined anthers of each 
flower, before the stigma of that individual flower is ready to 
receive them; and as this flower is never visited, at least in 
my garden, by insects, it never sets a seed, though by placing 
pollen from one flower on the stigma of another, I raise 
plenty of seedlings. Another species of Lobelia, which is 
visited by bees, seeds freely in my garden. In very many 
other cases, though there is no special mechanical contrivance 
to prevent the stigma receiving pollen from the same flower, 
yet, as Sprengel, and more recently Hildebrand, and others, 
have shown, and as I can confirm, either the anthers burst be- 
fore the stigma is ready for fertilisation, or the stigma is 


Teady before the pollen of that flower is ready, so that these 
so-named dichogamous plants have in fact separated sexes, 
and must habitually be crossed. So it is with the reciprocally 
dimorphic and trimorphic plants previously alluded to. How 
strange are these facts ! How strange that the pollen and 
stigmatic surface of the same flower, though placed so close 
together, as if for the very purpose of self-fertilisation, 
should be in so many cases mutually useless to each other? 
How simply are these facts explained on the view of an oc- 
casional cross with a distinct individual being advantageous 
or indispensable ! 

If several varieties of the cabbage, radish, onion, and of 
some other plants, be allowed to seed near each other, a large 
majority of the seedlings thus raised turn out, as I have 
found, mongrels : for instance, I raised 233 seedling cabbages 
from some plants of different varieties growing near each 
other, and of these only 78 were true to their kind, and some 
even of these were not perfectly true. Yet the pistil of each 
cabbage-flower is surrounded not only by its own six stamens 
but by those of the many other flowers on the same plant; 
and the pollen of each flower readily gets on its own stigma 
without insect agency ; for I have found that plants carefully 
protected from insects produce the full number of pods. 
How, then, comes it that such a vast number of the seedlings 
are mongrelized? It must arise from the pollen of a dis- 
tinct variety having a prepotent effect over the flower's own 
pollen ; and that this is part of the general law of good being 
derived from the intercrossing of distinct individuals of the 
same species. When distinct species are crossed the case is 
reversed, for a plant's own pollen is almost always prepotent 
over foreign pollen; but to this subject we shall return in a 
future chapter. 

In the case of a large tree covered with innumerable 
flowers, it may be objected that pollen could seldom be carried 
from tree to tree, and at most only from flower to flower on 
the sanre tree ; and flowers on the same tree can be consid- 
ered as distinct individuals only in a limited sense. I believe 
this objection to be valid, but that nature has largely pro- 
vided against it by giving to trees a strong tendency to bear 
flowers with separated sexes. When the sexes are separated, 


although the male and female flowers may be produced on the 
same tree, pollen must be regularly carried from flower to 
flower; and this will give a better chance of pollen being oc- 
casionally carried from tree to tree. That trees belonging to 
all Orders have their sexes more often separated than other 
plants, I find to be the case in this country ; and at my re- 
quest Dr. Hooker tabulated the trees of New Zealand, and 
Dr. Asa Gray those of the United States, and the result was 
as I anticipated. On the other hand, Dr. Hooker informs me 
that the rule does not hold good in Australia: but if most of 
the Australian trees are dichogamous, the same result would 
follow as if they bore flowers with separated sexes. I have 
made these few remarks on trees simply to call attention to 
the subject. 

Turning for a brief space to animals : various terrestrial 
species are hermaphrodites, such as the land-mollusca and 
earth-worms ; but these all pair. As yet I have not found a 
single terrestrial animal which can fertilise itself. This re- 
markable fact, which offers so strong a contrast with terres- 
trial plants, is intelligible on the view of an occasional cross 
being indispensable ; for owing to the nature of the fertilis- 
ing element there are no means, analogous to the action of 
insects and of the wind with plants, by which an occasional 
cross could be effected with terrestrial animals without the 
concurrence of two individuals. Of aquatic animals, there 
are many self-fertilising hermaphrodites; but here the cur- 
rents of water offer an obvious means for an occasional cross. 
As in the case of flowers, I have as yet failed, after consulta- 
tion with one of the highest authorities, namely. Professor 
Huxley, to discover a single hermaphrodite animal with the 
organs of reproduction so perfectly enclosed that access from 
•without, and the occasional influence of a distinct individual, 
can be shown to be physically impossible. Cirripedes long 
appeared to me to present, under this point of view, a case 
of great difficulty; but I have been enabled, by a fortunate 
chance, to prove that two individuals, though both are self- 
fertilising hermaphrodites, do sometimes cross. 

It must have struck most naturalists as a strange anomaly 
that, both with animals and plants, some species of the same 
family and even of the same genus, though agreeing closely 


with each other in their whole organisation, are hermaphro- 
dites, and some unisexual. But if, in fact, all hermaphro- 
dites do occasionally intercross, the difference between them 
and unisexual species is, as far as function is concerned, very- 

From these several considerations and from the many 
special facts which I have collected, but which I am unable 
here to give, it appears that with animals and plants an oc- 
casional intercross between distinct individuals is a very gen- 
eral, if not universal, law of nature. 


This is an extremely intricate subject. A great amount of 
variability, under which term individual differences are al- 
ways included, will evidently be favourable. A large num- 
ber of individuals, by giving a better chance within any given 
period for the appearance of profitable variations, will com- 
pensate for a lesser amount of variability in each individual, 
and is, I believe, a highly important element of success. 
Though Nature grants long periods of time for the work of 
natural selection, she does not grant an indefinite period; for 
as all organic beings are striving to seize on each place in 
the economy of nature, if any one species does not become 
modified and improved in a corresponding degree with its 
competitors, it will be exterminated. Unless favourable vari- 
ations be inherited by some at least of the offspring, nothing 
can be effected by natural selection. The tendency to rever- 
sion may often check or prevent the work; but as this ten- 
dency has not prevented man from forming by selection nu- 
merous domestic races, why should it prevail against natural 

In the case of methodical selection, a breeder selects for 
some definite object, and if the individuals be allowed freely 
to intercross, his work will completely fail. But when many 
men, without intending to alter the breed, have a nearly com- 
mon standard of perfection, and all try to procure and breed 
from the best animals, improvement surely but slowly follows 
from this unconscious process of selection, notwithstanding 


that there is no separation of selected individuals. Thus it 
will be under nature; for within a confined area, with some 
place in the natural polity not perfectly occupied, all the in- 
dividuals varying in the right direction, though in different 
degrees, will tend to be preserved. But if the area be large, 
its several districts will almost certainly present different con- 
ditions of life; and then, if the same species undergoes modi- 
fication in different districts, the newly-formed varieties will 
intercross on the confines of each. But we shall see in the 
sixth chapter that intermediate varieties, inhabiting inter- 
mediate districts, will in the long run generally be supplanted 
by one of the adjoining varieties. Intercrossing will chiefly 
affect those animals which unite for each birth and wander 
much, and which do not breed at a very quick rate. Hence 
with animals of this nature, for instance, birds, varieties will 
generally be confined to separated countries ; and this I find 
to be the case. With hermaphrodite organisms which cross 
only occasionally, and likewise with animals which unite for 
each birth, btat which wander little and can increase at a 
rapid rate, a new and improved variety might be quickly 
formed on any one spot, and might there maintain itself in a 
body and afterwards spread, so that the individuals of the 
new variety would chiefly cross together. On this principle, 
nurserymen always prefer saving seed from a large body of 
plants, as the chance of intercrossing is thus lessened. 

Even with animals which unite for each birth, and which 
do not propagate rapidly, we must not assume that free in- 
tercrossing would always eliminate the effects of natural 
selection ; for I can bring forward a considerable body of 
facts showing that within the same area, two varieties of the 
same animal may long remain distinct, from haunting differ- 
ent stations, from breeding at slightly different seasons, or 
from the individuals of each variety preferring to pair to- 

Intercrossing plays a very important part in nature by 
keeping the individuals of the same species, or of the same 
variety, true and uniform in character. It will obviously 
thus act far more efficiently with those animals which unite 
for each birth ; but, as already stated, we have reason to be- 
lieve that occasional intercrosses take place with all animals 


and plants. Even if these take place only at long intervals 
of time, the young thus produced will gain so much in vigour 
and fertility over the offspring from long-continued self-fer- 
tilisation, that they vi^ill have a better chance of surviving 
and propagating their kind ; and thus in the long run the in- 
fluence of crosses, even at rare intervals, will be great. With 
respect to organic beings extremely low in the scale, which 
do not propagate sexually, nor conjugate, and which cannot 
possibly intercross, uniformity of character can be retained 
by them under the same conditions of life, only through the 
principle of inheritance, and through natural selection which 
will destroy any individuals departing from the proper type. 
If the conditions of life change and the form undergoes modi- 
fication, uniformity of character can be given to the modified 
offspring, solely by natural selection preserving similar fa- 
vourable variations. 

Isolation, also, is an important element in the modification 
of species through natural selection. In a confined or iso- 
lated area, if not very large, the organic and inorganic con- 
ditions of life will generally be almost uniform; so that nat- 
ural selection will tend to modify all the varying individuals 
of the same species in the same manner. Intercrossing with 
the inhabitants of the surrounding districts will, also, be thus 
prevented. Moritz Wagner has lately published an interest- 
ing essay on this subject, and has shown that the service 
rendered by isolation in preventing crosses between newly- 
formed varieties is probably greater even than I supposed. 
But from reasons already assigned I can by no means agree 
with this naturalist, that migration and isolation are neces- 
sary elements for the formation of new species. The im- 
portance of isolation is likewise great in preventing, after 
any physical change in the conditions such as of climate ele- 
vation of the land, &c., the immigration of better adapted or- 
ganisms; and thus new places in the natural economy of the 
district will be left open to be filled up by the modification of 
the old inhabitants. Lastly, isolation will give time for a 
new variety to be improved at a slow rate ; and this may 
sometimes be of much importance. If, however, an isolated 
area be very small, either from being surrounded by barriers, 
or from having very peculiar physical conditions, the total 


number of the inhabitants will be small; and this will retard 
the production of new species through natural selection, by 
decreasing the chances of favourable variations arising. 

The mere lapse of time by itself does nothing, either for 
or against natural selection. I state this because it has been 
erroneously asserted that the element of time has been as- 
sumed by me to play an all-important part in modifying 
species, as if all the forms of life were necessarily undergo- 
ing change through some innate law. Lapse of time is only 
so far important, and its importance in this respect is great, 
that it gives a better chance of beneficial variations arising 
and of their being selected, accumulated, and fixed. It like- 
wise tends to increase the direct action of the physical 
conditions of life, in relation to the constitution of each 

If we turn to nature to test the truth of these remarks, and 
look at any small isolated area, such as an oceanic island, al- 
though the number of species inhabiting it is small, as we 
shall see in our chapter on Geographical Distribution ; yet 
of these species a very large proportion are endemic, — that 
is, have been produced there and nowhere else in the world. 
Hence an oceanic island at first sight seems to have been 
highly favourable for the production of new species. But 
we may thus deceive ourselves, for to ascertain whether a 
small isolated area, or a large open area like a continent, has 
been most favourable for the production of new organic 
forms, we ought to make the comparison within equal times ; 
and this we are incapable of doing. 

Although isolation is of great importance in the production 
of new species, on the whole I am inclined to believe that 
largeness of area is still more important, especially for the 
production of species which shall prove capable of enduring 
for a long period, and of spreading widely. Throughout a 
great and open area, not only will there be a better chance of 
favourable variations, arising from the large number of indi- 
viduals of the same species there supported, but the conditions 
of life are much more complex from the large number of al- 
ready existing species; and if some of these many species 
become modified and improved, others will have to be im- 
proved in a corresponding degree, or they will be extermi- 


nated. Each new form, also, as soon as it has been much 
improved, will be able to spread over the open and continu- 
ous area, and will thus come into competition with many- 
other forms. Moreover, great areas, though now continuous, 
will often, owing to former oscillations of level, have existed 
in a broken condition; so that the good effects of isolation 
will generally, to a certain extent, have concurred. Finally, 
I conclude that, although small isolated areas have been in 
some respects highly favourable for the production of new 
species, yet that the course of modification will generally have 
been more rapid on large areas ; and what is more important, 
that the new forms produced on large areas, which already 
have been victorious over many competitors, will be those 
that will spread most widely, and will give rise to the great- 
est number of new varieties and species. They will thus 
play a more important part in the changing history of the 
organic world. 

In accordance with this view, we can, perhaps, understand 
some facts which will be again alluded to in our chapter on 
Geographical Distribution; for instance, the fact of the pro- 
ductions of the smaller continent of Australia now yielding 
before those of the larger Europaeo-Asiatic area. Thus, also, 
it is that continental productions have everywhere become so 
largely naturalised on islands. On a small island, the race 
for life will have been less severe, and there will have been 
less modification and less extermination. Hence, we can 
•understand how it is that the flora of Madeira, according to 
Oswald Heer, resembles to a certain extent the extinct ter- 
tiary flora of Europe. All fresh-water basins, taken together, 
make a small area compared with that of the sea or of the 
land. Consequently, the competition between fresh-water 
productions will have been less severe than elsewhere; new 
forms will have been then more slowly produced, and old 
forms more slowly exterminated. And it is in fresh-water 
basins that we find seven genera of Ganoid fishes, remnants 
of a once preponderant order : and in fresh water we find 
some of the most anomalous forms now known in the world 
as the Ornithorhynchus and Lepidosiren, which, like fossils, 
connect to a certain extent orders at present widely sundered 
in the natural scale. These anomalous forms may be called 


living fossils ; they have endured to the present day, from 
having inhabited a confined area, and from having been ex- 
posed to less varied, and therefore less severe, competition. 

To sum up, as far as the extreme intricacy of the subject 
permits, the circumstances favourable and unfavourable for 
the production of new species through natural selection. I 
conclude that for terrestrial productions a large continental 
area, which has undergone many oscillations of level, will 
have been the most favourable for the production of many 
new forms of life, fitted to endure for a long time and to 
spread widely. Whilst the area existed as a continent, the in- 
habitants will have been numerous in individuals and kinds, 
and will have been subjected to severe competition. When 
converted by subsidence into large separate islands, there 
will still have existed many individuals of the same species 
on each island; intercrossing on the confines of the range of 
each new species will have been checked ; after physical 
changes of any kind, immigration will have been prevented, 
so that new places in the polity of each island will have had 
to be filled up by the modification of the old inhabitants ; and 
time will have been allowed for the varieties in each to be- 
come well modified and perfected. When, by renewed eleva- 
tion, the islands were reconverted into a continental area, 
there will again have been very severe competition : the most 
favoured or improved varieties will have been enabled to 
spread : there will have been much extinction of the less im- 
proved forms, and the relative proportional numbers of the 
various inhabitants of the reunited continent will again have 
been changed ; and again there will have been a fair field for 
natural selection to improve still further the inhabitants, and 
thus to produce new species. 

That natural selection generally acts with extreme slow- 
ness I fully admit. It can act only when there are places in 
the natural polity of a district which can be better occupied 
by the modification of some of its existing inhabitants. The 
occurrence of such places will often depend on physical 
changes, which generally take place very slowly, and on the 
immigration of better adapted forms being prevented. As 
some few of the old inhabitants become modified, the mutual 
relations of others will often be disturbed; and this will 


create new places, ready to be filled up by better adapted 
forms ; but all this will take place very slowly. Although all 
the individuals of the same species differ in some slight de- 
gree from each other, it would often be long before differ- 
ences of the right nature in various parts of the organisation 
might occur. The result would often be greatly retarded by 
free intercrossing. Many will exclaim that these several 
causes are amply sufficient to neutralise the power of nat- 
ural selection. I do not believe so. But I do believe that 
natural selection will generally act very slowly, only at long 
intervals of time, and only on a few of the inhabitants of the 
same region. I further believe that these slow, intermittent 
results accord well with what geology tells us of the rate and 
manner at which the inhabitants of the world have changed. 
Slow though the process of selection may be, if feeble man 
can do much by artificial selection, I can see no limit to the 
amount of change, to the beauty and complexity of the co- 
adaptations between all organic beings, one with another 
and with their physical conditions of life, which may have 
been affected in the long course of time through nature's 
power of selection, that is by the survival of the fittest. 


This subject will be more fully discussed in our chapter on 
Geology; but it must here be alluded to from being inti- 
mately connected with natural selection. Natural selection 
acts solely through the preservation of variations in some 
way advantageous, which consequently endure. Owing to 
the high geometrical rate of increase of all organic beings, 
each area is already fully stocked with inhabitants; and it 
follows from this, that as the favoured forms increase in 
number, so, generally, will the less favoured decrease and 
become rare. Rarity, as geology tells us, is the precursor to 
extinction. We can see that any form which is represented 
by few individuals will run a good chance of utter extinc- 
tion, during great fluctuations in the nature of the seasons, 
or from a temporary increase in the number of its enemies. 
But we may go further than this ; for, as new forms are pro- 
duced, unless we admit that specific forms can go on indefi- 


nitely increasing in number, many old forms must become ex- 
tinct. That the number of specific forms has not indefinitely 
increased, geology plainly tells us; and we shall presently at- 
tempt to show why it is that the number of species through- 
out the world has not become immeasurably great. 

We have seen that the species which are most numerous 
in individuals have the best chance of producing favourable 
variations within any given period. We have evidence of 
this, in the facts stated in the second chapter, showing that 
it is the common and diffused or dominant species which 
offer the greatest number of recorded varieties. Hence, rare 
species will be less quickly modified or improved within any 
given period ; they will consequently be beaten in the race for 
life by the modified and improved descendants of the com- 
moner species. 

From these several considerations I think it inevitably fol- 
lows, that as new species in the course of time are formed 
through natural selection, others will become rarer and rarer, 
and finally extinct. The forms which stand in closest com- 
petition with those undergoing modification and improve- 
ment, will naturally suffer most. And we have seen in the 
chapter on the Struggle for Existence that it is the most 
closely-allied forms, — varieties of the same species, and 
species of the same genus or of related genera, — which, from 
having nearly the same structure, constitution, and habits, 
generally come into the severest competition with each 
other ; consequently, each new variety or species, during the 
progress of its formation, will generally press hardest on its 
nearest kindred, and tend to exterminate them. We see the 
same process of extermination amongst our domesticated pro- 
ductions, through the selection of improved forms by man. 
Many curious instances could be given showing how quickly 
new breeds of cattle, sheep, and other animals, and varieties 
of flowers, take the place of older and inferior kinds. In 
Yorkshire, it is historically known that the ancient black 
cattle were displaced by the long-horns, and that these "were 
swept away by the short-horns" (I quote the words of an 
agricultural writer) "as if by some murderous pestilence." 



The principle, which I have designated by this term, is of 
high importance, and explains, as I believe, several impor- 
tant facts. In the first place, varieties, even strongly-marked 
ones, though having somewhat of the character of species — 
as is shown by the hopeless doubts in many cases how to 
rank them — yet certainly differ far less from each other than 
do good and distinct species. Nevertheless, according to my 
view, varieties are species in the process of formation, or are, 
as I have called them, incipient species. How, then, does 
the lesser difference between varieties become augmented into 
the greater difference between species? That this does habit- 
ually happen, we must infer from most of the innumerable 
species throughout nature presenting well-marked differ- 
ences ; whereas varieties, the supposed prototypes and par- 
ents of future well-marked species, present slight and ill-de- 
fined differences. Mere chance, as we may call it, might 
cause one variety to differ in some character from its parents, 
and the offspring of this variety again to differ from its 
parent in the very same character and in a greater degree; 
but this alone would never account for so habitual and large 
a degree of difference as that between the species of the same 

As has always been my practice, I have sought light on this 
head from our domestic productions. We shall here find 
something analogous. It will be admitted that the production 
of races so different as short-horn and Hereford cattle, race 
and cart horses, the several breeds of pigeons, &c., could 
never have been effected by the mere chance accumulation of 
similar variations during many successive generations. In 
practice, a fancier is, for instance, struck by a pigeon having 
a slightly shorter beak ; another fancier is struck by a pigeon 
having a rather longer beak; and on the acknowledged 
principle that "fanciers do not and will not admire a me- 
dium standard, but like extremes," they both go on (as 
has actually occurred with the sub-breeds of the tumbler- 
pigeon) choosing and breeding from birds with longer and 
longer beaks, or with shorter and shorter beaks. Again, we 
may suppose that at an early period of history, the men of 


one nation or district required swifter horses, whilst those of 
another required stronger and bulkier horses. The early dif- 
ferences would be very slight ; but, in the course of time, from 
the continued selection of swifter horses in the one case, and 
of stronger ones in the other, the differences would become 
greater, and would be noted as forming two sub-breeds. Ul- 
timately, after the lapse of centuries, these sub-breeds would 
become converted into two well-established and distinct 
breeds. As the differences became greater, the inferior ani- 
mals with intermediate characters, being neither very swift 
nor very strong, would not have been used for breeding, and 
will thus have tended to disappear. Here, then, we see in 
man's productions the action of what may be called the prin- 
ciple of divergence, causing differences, at first barely appre- 
ciable, steadily to increase, and the breeds to diverge in 
character, both from each other and from their common 

But how, it may be asked, can any analogous principle 
apply in nature ? I believe it can and does apply most effi- 
ciently (though it was a long time before I saw how), from 
the simple circumstance that the more diversified the de- 
scendants from any one species become in structure, consti- 
tution, and habits, by so much will they be better enabled to 
seize on many and widely diversified places in the polity of 
nature, and so be enabled to increase in numbers. 

We can clearly discern this in the case of animals with 
simple habits. Take the case of a carnivorous quadruped, 
of which the number that can be supported in any country has 
long ago arrived at its full average. If its natural power of 
increase be allowed to act, it can succeed in increasing (the 
country not undergoing any change in conditions) only by 
its varying descendants seizing on places at present occupied 
by other animals; some of them, for instance, being enabled 
to feed on new kinds of prey, either dead or alive ; some 
inhabiting new stations, climbing trees, frequenting water, 
and some perhaps becoming less carnivorous. The more 
diversified in habits and structure the descendants of our 
carnivorous animals become, the more places they will be 
enabled to occupy. What applies to one animal will apply 
throughout all time to all animals — that is, if they vary — for 


otherwise natural selection can effect nothing. So it will be 
with plants. It has been experimentally proved, that if a 
plot of ground be sown with one species of grass, and a similar 
plot be sown with several distinct genera of grasses, a greater 
number of plants and a greater weight of dry herbage can 
be raised in the latter than in the former case. The same 
has been found to hold good when one variety and several 
mixed varieties of wheat have been sown on equal spaces of 
ground. Hence, if any one species of grass were to go on 
varying, and the varieties were continually selected which 
differed from each other in the same manner, though in a 
very slight degree, as do the distinct species and genera of 
grasses, a greater number of individual plants of this species, 
including its modified descendants, would succeed in living 
on the same piece of ground. And we know that each 
species and each variety of grass is annually sowing almost 
countless seeds ; and is thus striving, as it may be said, to the 
utmost to increase in number. Consequently, in the course of 
many thousand generations, the most distinct varieties of 
any one species of grass would have the best chance of suc- 
ceeding and of increasing in numbers, and thus of supplanting 
the less distinct varieties ; and varieties, when rendered very 
distinct from each other, take the rank of species. 

The truth of the principle that the greatest amount of life 
can be supported by great diversification of structure, is seen 
under many natural circumstances. In an extremely small 
area, especially if freely open to immigration, and where the 
contest between individual and individual must be very se- 
vere, we always find great diversity in its inhabitants. For 
instance, I found that a piece of turf, three feet by four 
in size, which had been exposed for many years to exactly 
the same conditions, supported twenty species of plants, and 
these belonged to eighteen genera and to eight orders, which 
shows how much these plants dififered from each other. So 
it is with the plants and insects on small and uniform islets: 
also in small ponds of fresh water. Farmers find that they 
can raise most food by a rotation of plants belonging to the 
most different orders; nature follows what may be called a 
simultaneous rotation. Most of the animals and plants which 
live close round any small piece of ground, could live on it 


(supposing its nature not to be in any way peculiar), and 
may be said to be striving to the utmost to live there ; but, it 
is seen, that where they come into the closest competition, the 
advantages of diversification of structure, with the accom- 
panying differences of habit and constitution, determine that 
the inhabitants, which thus jostle each other most closely, 
shall, as a general rule, belong to what we call different 
genera and orders. 

The same principle is seen in the naturalisation of plants 
through man's agency in foreign lands. It might have been 
expected that the plants which would succeed in becoming 
naturalised in any land would generally have been closely 
allied to the indigenes; for these are commonly looked at as 
specially created and adapted for their own country. It 
might also, perhaps, have been expected that naturalised 
plants would have belonged to a few groups more especially 
adapted to certain stations in their new homes. But the 
case is very different; and Alph. de Candolle has well re- 
marked, in his great and admirable work, that floras gain by 
naturalisation, proportionally with the number of the native 
genera and species, far more in new genera than in new 
species. To give a single instance: in the last edition of 
Dr. Asa Gray's 'Manual of the Flora of the Northern United 
States,' 260 naturalised plants are enumerated, and these be- 
long to 162 genera. We thus see that these naturalised plants 
are of a highly diversified nature. They differ, moreover, to 
a large extent, from the indigenes, for out of the 162 natural- 
ised genera, no less than 100 genera are not there indigenous, 
and thus a large proportional addition is made to the genera 
now living in the United States. 

By considering the nature of the plants or animals which 
have in any country struggled successfully with the indigenes, 
and have there become naturalised, we may gain some crude 
idea in what manner some of the natives would have to be 
modified, in order to gain an advantage over their com- 
patriots; and we may at least infer that diversification of 
structure, amounting to new generic differences, would be 
profitable to them. 

The advantage of diversification of structure in the in- 
habitants of the same region is, in fact, the same as that of 


the physiological division of labor in the organs of the same 
individual body — a subject so well elucidated by Milne Ed- 
wards. No physiologist doubts that a stomach adapted to 
digest vegetable matter alone, or flesh alone, draws most 
nutriment from these substances. So in the general economy 
of any land, the more widely and perfectly the animals and 
plants are diversified for different habits of life, so will a 
greater number of individuals be capable of there supporting 
themselves. A set of animals, with their organisation but 
little diversified, could hardly compete with a set more per- 
fectly diversified in structure. It may be doubted, for in- 
stance, whether the Australian marsupials, which are divided 
into groups differing but little from each other, and feebly 
representing, as Mr. Waterhouse and others have remarked, 
our carnivorous, ruminant, and rodent mammals, could suc- 
cessfully compete with these well-developed orders. In the 
Australian mammals, we see the process of diversification 
in an early and incomplete stage of development. 


After the foregoing discussion, which has been much com- 
pressed, we may assume that the modified descendants of any 
one species will succeed so much the better as they become 
more diversified in structure, and are thus enabled to en- 
croach on places occupied by other beings. Now let us see 
how this principle of benefit being derived from divergence 
of character, combined with the principles of natural selec- 
tion and of extinction, tends to act. 

The accompanying diagram will aid us in understanding 
this rather perplexing subject. Let A to L represent the 
species of a genus large in its own country ; these species are 
supposed to resemble each other in unequal degrees, as is 
so generally the case in nature, and as is represented in the 
diagram by the letters standing at unequal distances. I have 
said a large genus, because as we saw in the second chapter, 
on an average more species vary in large genera than in 


small genera; and the varying species of the large genera 
present a greater number of varieties. We have, also, seen 
that the species, which are the commonest and the most 
widely diffused, vary more than do the rare and restricted 
species. Let (A) be a common, widely-diffused, and varying 
species, belonging to a genus large in its own country. The 
branching and diverging dotted lines of unequal lengths pro- 
ceeding from (A), may represent its varying offspring. The 
variations are supposed to be extremely slight, but of the 
most diversified nature; they are not supposed all to appear 
simultaneously, but often after long intervals of time ; nor 
are they all supposed to endure for equal periods. Only those 
variations which are in some way profitable will be preserved 
or naturally selected. And here the importance of the prin- 
ciple of benefit derived from divergence of character comes 
in ; for this will generally lead to the most different or di- 
vergent variations (represented by the outer dotted lines) 
being preserved and accumulated by natural selection. When 
a dotted line reaches one of the horizontal lines, and is there 
marked by a small numbered letter, a sufficient amount of 
variation is supposed to have been accumulated to form it 
into a fairly well-marked variety, such as would be thought 
worthy of record in a systematic work. 

The intervals between the horizontal lines in the diagram, 
may represent each a thousand or more generations. After a 
thousand generations, species (A) is supposed to have pro- 
duced two fairly well-marked varieties, namely a* and w\ 
These two varieties will generally still be exposed to the 
same conditions which made their parents variable, and the 
tendency to variability is in itself hereditary ; consequently 
they will likewise tend to vary, and commonly in nearly the 
same manner as did their parents. Moreover, these two 
varieties, being only slightly modified forms, will tend to 
inherit those advantages which made their parent (A) more 
numerous than most of the other inhabitants of the same 
country; they will also partake of those more general advan- 
tages which made the genus to which the parent-species 
belonged, a large genus in its own country. And all 
these circumstances are favorable to the production of new 


If, then, these two varieties be variable, the most divergent 
of their variations will generally be preserved during the 
next thousand generations. And after this interval, variety 
a^ is supposed in the diagram to have produced variety a", 
which will, owing to the principle of divergence, differ more 
from (A) than did variety a.\ Variety m^ is supposed to 
have produced two varieties, namely m^ and /^ differing from 
each other, and more considerably from their common parent 
(A). We may continue the process by similar steps for any 
length of time; some of the varieties, after each thousand 
generations, producing only a single variety, but in a more 
and more modified condition, some producing two or three 
varieties, and some failing to produce any. Thus the varie- 
ties or modified descendants of the common parent (A), will 
generally go on increasing in number and diverging in char- 
acter. In the diagram the process is represented up to the 
ten-thousandth generation, and under a condensed and sim- 
plified form up to the fourteen-thousandth generation. 

But I must here remark that I do not suppose that the 
process ever goes on so regularly as is represented in the 
diagram, though in itself made somewhat irregular, nor that 
it goes on continuously; it is far more probable that each 
form remains for long periods unaltered, and then again 
undergoes modification. Nor do I suppose that the most di- 
vergent varieties are invariably preserved; a medium form 
may often long endure, and may or may not produce more 
than one modified descendant; for natural selection will al- 
ways act according to the nature of the places which are 
either unoccupied or not perfectly occupied by other beings; 
and this will depend on infinitely complex relations. But as 
a general rule, the more diversified in structure the descend- 
ants from any one species can be rendered, the more places 
they will be enabled to seize on, and the more their modified 
progeny will increase. In our diagram the line of succession 
is broken at regular intervals by small numbered letters mark- 
ing the successive forms which have become sufficiently dis- 
tinct to be recorded as varieties. But these breaks are 
imaginary, and might have been inserted anywhere, after 
intervals long enough to allow the accumulation of a con- 
siderable amount of divergent variation. 


As all the modified descendants from a common and widely- 
diffused species, belonging to a large genus, will tend to par- 
take of the same advantages which made their parent success- 
ful in life, they will generally go on multiplying in number as 
well as diverging in character; this is represented in the dia- 
gram by the several divergent branches proceeding from (A). 
The modified offspring from the later and more highly im- 
proved branches in the lines of descent, will, it is probable, 
often take the place of, and so destroy, the earlier and less im- 
proved branches : this is represented in the diagram by some 
of the lower branches not reaching to the upper horizontal 
lines. In some cases no doubt the process of modification 
will be confined to a single line of descent, and the number 
of modified descendants will not be increased; although the 
amount of divergent modification may have been augmented. 
This case would be represented in the diagram, if all the 
lines proceeding from (A) were removed, excepting that 
from a^ to a". In the same way the English race-horse and 
English pointer have apparently both gone on slowly diverg- 
ing in character from their original stocks, without either 
having given off any fresh branches or races. 

After ten thousand generations, species (A) is supposed 
to have produced three forms, o", f and m", which, from 
having diverged in character during the successive genera- 
tions, will have come to differ largely, but perhaps unequally, 
from each other and from their common parent. If we sup- 
pose the amount of change between each horizontal line in 
our diagram to be excessively small, these three forms may 
still be only well-marked varieties ; but we have only to 
suppose the steps in the process of modification to be more 
numerous or greater in amount, to convert these three forms 
into doubtful or at least into well-defined species. Thus the 
diagram illustrates the steps by which the small differences 
distinguishing varieties are increased into the larger differ- 
ences distinguishing species. By continuing the same process 
for a greater number of generations (as shown in the dia- 
gram in a condensed and simplified manner), we get eight 
species, marked by the letters between o" and »i", all de- 
scended from (A). Thus, as I believe, species are multiplied 
and genera are formed. 



In a large genus it is probable that more than one species 
would vary. In the diagram I have assumed that a second 
species (I) has produced, by analogous steps, after ten thou- 
sand generations, either two well-marked varieties (w" and 
z^") or two species, according to the amount of change sup- 
posed to be represented between the horizontal lines. After 
fourteen thousand generations, six new species, marked by 
the letters n" to z^*, are supposed to have been produced. In 
any genus, the species which are already very different in 
character from each other, will generally tend to produce the 
greatest number of modified descendants ; for these will have 
the best chance of seizing on new and widely different places 
in the polity of nature: hence in the diagram I have choeen 
the extreme species (A), and the nearly extreme species (I), 
as those which have largely varied, and have given rise to 
new varieties and species. The other nine species (marked 
by capital letters) of our original genus, may for long but 
unequal periods continue to transmit unaltered descendants; 
and this is shown in the diagram by the dotted lines unequally 
prolonged upwards. 

But during the process of modification, represented in the 
diagram, another of our principles, namely that of extinction, 
will have played an important part. As in each fully stocked 
country natural selection necessarily acts by the selected 
form having some advantage in the struggle for life over 
other forms, there will be a constant tendency in the im- 
proved descendants of any one species to supplant and ex- 
terminate in each stage of descent their predecessors and 
their original progenitor. For it should be remembered that 
the competition will generally be most severe between those 
forms which are most nearly related to each other in habits, 
constitution, and structure. Hence all the intermediate forms 
between the earlier and later states^ that is between the less 
and more improved states of the same species, as well as 
the original parent-species itself, will generally tend to become 
extinct. So it probably will be with many whole collateral 
lines of descent which will be conquered by later and 
improved lines. If, however, the modified offspring of a 
species get into some distinct country, or become quickly 
adapted to some quite new station, in which oflfspring and 


progenitor do not come into competition, both may continue 
to exist. 

If, then, our diagram be assumed to represent a consider- 
able amount of modification, species (A) and all the earlier 
varieties will have become extinct, being replaced by eight 
new species (a" to m") ; and species (I) will be replaced by 
six (n" to 2") new species. 

But we may go further than this. The original species 
of our genus were supposed to resemble each other in unequal 
degrees, as is so generally the case in nature; species (A) 
being more nearly related to B, C, and D, than to the other 
species; and species (I) more to G, H, K, L, than to the 
others. These two species (A) and (I) were also supposed 
to be very common and widely dififused species, so that they 
must originally have had some advantage over most of the 
other species of the genus. Their modified descendants, 
fourteen in number at the fourteen-thousandth generation, 
will probably have inherited some of the same advantages: 
they have also been modified and improved in a diversified 
"manner at each stage of descent, so as to have become adapted 
to many related places in the natural economy of their 
country. It seems, therefore, extremely probable that they 
will have taken the places of, and thus exterminated, not only 
their parents (A) and (I), but likewise some of the original 
species which were most nearly related to their parents. 
Hence very few of the original species will have transmitted 
offspring to the fourteen-thousandth generation. We may 
suppose that only one, (F), of the two species (E and F) 
which were least closely related to the other nine original 
species, has transmitted descendants to this late stage of 

The new species in our diagram descended from the original 
eleven species, will now be fifteen in number. Owing to the 
divergent tendency of natural selection, the extreme amoimt 
of difference in character between species o" and ^r" will be 
much greater than that between the most distinct of the 
original eleven species. The new species, moreover, will be 
allied to each other in a widely different manner. Of the 
eight descendants from (A) the three marked fl", 7", />'*, 
will be nearly related from having recently branched off 


from d"; &", and f*, from having diverged at an earlier 
period from a^ will be in some degree distinct from the three 
first-named species; and lastly, o", i"" and w" will be nearly 
related one to the other, but, from having diverged at the first 
commencement of the process of modification, will be widely 
different from the other five species, and may constitute a 
sub-genus or a distinct genus. 

The six descendants from (I) will form two sub-genera or 
genera. But as the original species (I) differed largely from 
(A), standing nearly at the extreme end of the original 
genus, the six descendants from (I) will, owing to inherit- 
ance alone, differ considerably from the eight descendants 
from (A) ; the two groups, moreover, are supposed to have 
gone on diverging in different directions. The intermediate 
species, also (and this is a very important consideration), 
which connected the original species (A) and (I), have all 
become, excepting (F), extinct, and have left no descend- 
ants. Hence the six new species descended from (I), and 
the eight descendants from (A), will have to be ranked as 
very distinct genera, or even as distinct sub-families. 

Thus it is, as I believe, that two or more genera are pro- 
duced by descent with modification, from two or more species 
of the same genus. And the two or more parent-species are 
supposed to be descended from some one species of an earlier 
genus. In our diagram, this is indicated by the broken lines, 
beneath the capital letters, converging in sub-branches down- 
wards towards a single point ; this point represents a species, 
the supposed progenitor of our several sub-genera and 

It is worth while to reflect for a moment on the character 
of the new species f", which is supposed not to have diverged 
much in character, but to have retained the form of (F), 
either unaltered or altered only in a slight degree. In this 
case, its affinities to the other fourteen new species will be of 
a curious and circuitous nature. Being descended from a 
form which stood between the parent-species (A) and (I), 
now supposed to be extinct and unknown, it will be in some 
degree intermediate in character between the two groups 
descended from these two species. But as these two groups 
have gone on diverging in character from the type of their 


parents, the new species (f") will not be directly interme- 
diate between them, but rather between types of the two 
groups; and every naturalist will be able to crJl su^h cases 
before his mind. 

In the diagram, each horizontal line has hitherto been sup- 
posed to represent a thousand generations, but each may rep- 
resent a million or more generations; it may also represent a 
section of the successive strata of the earth's crust including 
extinct remains. We shall, when we come to our chapter on 
Geolog>', have to refer again to this subject, and I think we 
shall then see that the diagram throws light on the affinities 
of extinct beings, which, though generally belonging to the 
same orders, families, or genera, with those now living, yet 
are often, in some degree, intermediate in character between 
existing groups ; and we can understand this fact, for the ex- 
tinct species lived at various remote epochs when the 
branching lines of descent had diverged less. 

I see no reason to limit the process of modification, as now 
explained, to the formation of genera alone. If, in the dia- 
gram, we suppose the amount of change represented by each 
successive group of diverging dotted lines to be great, the 
forms marked o" to />", those marked &" and f *, and those 
marked o" to m", will form three very distinct genera. We 
shall also have two very distinct genera descended from (I), 
differing widely from the descendants of (A). Those two 
groups of genera will thus form. two distinct families, or 
orders, according to the amount of divergent modification 
supposed to be represented in the diagram. And the two new 
families, or orders, are descended from two species of the 
original genus, and these are supposed to be descended from 
some still more ancient and unknown form. 

We have seen that in each country it is the species belong- 
ing to the larger genera which oftenest present varieties or 
incipient species. This, indeed, might have been expected ; 
for, as natural selection acts through one form having some 
advantage over other forms in the struggle for existence, it 
will chiefly act on those which already have some advantage ; 
and the largeness of any group shows that its species have 
inherited from a common ancestor some advantage in com- 
mon. Hence, the struggle for the production of new and 


modified descendants will mainly lie between the larger 
groups which are all trying to increase in number. One 
large group will slowly conquer another large group, reduce 
its numbers, and thus lessen its chance of further variation 
and improvement. Within the same large group, the later 
and more highly perfected sub-groups, from branching out 
and seizing on many new places in the polity of Nature, will 
constantly tend to supplant and destroy the earlier and less 
improved sub-groups. Small and broken groups and sub- 
groups will finally disappear. Looking to the future, we can 
predict that the groups of organic beings which are now 
large and triumphant, and which are least broken up, that 
is, which have as yet suffered least extinction, will, for a long 
period, continue to increase. But which groups will ulti- 
mately prevail, no man can predict; for we know that many 
groups, formerly most extensively developed, have now be- 
come extinct. Looking still more remotely to the future, we 
may predict that, owing to the continued and steady increase 
of the larger groups, a multitude of smaller groups will 
become utterly extinct, and leave no modified descendants; 
and consequently that, of the species living at any one period, 
extremely few will transmit descendants to a remote futurity. 
I shall have to return to this subject in the chapter on Classi- 
fication, but I may add that as, according to this view, ex- 
tremely few of the more ancient species have transmitted 
descendants to the present day, and, as all the descendants 
of the same species form a class, we can understand how it 
is that there exist so few classes in each main division of 
the animal and vegetable kingdoms. Although few of the 
most ancient species have left modified descendants, yet, at 
remote geological periods, the earth may have been almost 
as well peopled with species of many genera, families, orders, 
and classes, as at the present time. 


Natural Selection acts exclusively by the preservation and 
accumulation of variations, which are beneficial under the 
organic and inorganic conditions to which each creature is 
exposed at all periods of life. The ultimate result is that each 


creature tends to become more and more improved in relation 
to its conditions. This improvement inevitably leads to the 
gradual advancement of the organisation of the greater num- 
ber of living beings throughout the world. But here we enter 
on a very intricate subject, for naturalists have not defined 
to each other's satisfaction what is meant by an advance in 
organization. Amongst the vertebrata the degree of intellect 
and an approach in structure to man clearly come into play. 
It might be thought that the amount of change which the 
various parts and organs pass through in their development 
from the embryo to maturity would suffice as a standard of 
comparison ; but there are cases, as with certain parasitic 
crustaceans, in which several parts of the structure become 
-less perfect, so that the mature animal cannot be called higher 
than its larva. Von Baer's standard seems the most widely 
applicable and the best, namely, the amount of differentiation 
of the parts of the same organic being, in the adult state as 
I should be inclined to add, and their specialisation for dif- 
ferent functions ; or, as Milne Edwards would express it, 
the completeness of the division of physiological labour. But 
we shall see how obscure this subject is if we look, for in- 
stance, to fishes, amongst which some naturalists rank those 
as highest which, like the sharks, approach nearest to amphi- 
bians; whilst other naturalists range the common bony or 
teleostean fishes as the highest, inasmuch as they are most 
strictly fish-like, and differ most from the other vertebrate 
classes. We see still more plainly the obscurity of the 
subject by turning to plants, amongst which the standard of 
intellect is of course quite excluded; and here some botanists 
rank those plants as highest which have every organ, as 
sepals, petals, stamens, and pistils, fully developed in each 
flower; whereas other botanists, probably with more truth, 
look at the plants which have their several organs much 
modified and reduced in number as the highest. 

If we take as the standard of high organisation, the amount 
of differentiation and specialisation of the several organs in 
each being when adult (and this will include the advance- 
ment of the brain for intellectual purposes), natural selec- 
tion clearly leads towards this standard: for all physiologists 
admit that the specialisation of organs, inasmuch as in this 


state they perform their functions better, is an advantage to 
each being; and hence the accumulation of variations tending 
towards specialisation is within the scope of natural selection. 
On the other hand, we can see, bearing in mind that all or- 
ganic beings are striving to increase at a high ratio and to 
seize on every unoccupied or less well occupied place in the 
economy of nature, that it is quite possible for natural selec- 
tion gradually to fit a being to a situation in which several 
organs would be superfluous or useless: in such cases there 
would be retrogression in the scale of organisation. Whether 
organisation on the whole has actually advanced from the 
remotest geological periods to the present day will be more 
conveniently discussed in our chapter on Geological Succes- 

But it may be objected that if all organic beings thus tend 
to rise in the scale, how is it that throughout the world a 
multitude of the lowest forms still exist; and how is it that 
in each great class some forms are far more highly developed 
than others ? Why have not the more highly developed forms 
everywhere supplanted and exterminated the lower? La- 
marck, who believed in an innate and inevitable tendency 
towards perfection in all organic beings, seems to have felt 
this difficulty so strongly, that he was led to suppose that new 
and simple forms are continually being produced by spon- 
taneous generation. Science has not as yet proved the truth 
of this belief, whatever the future may reveal. On our 
theory the continued existence of lowly organisms offers no 
difficulty; for natural selection, or the survival of the fittest, 
does not necessarily include progressive development — it 
only takes advantage of such variations as arise and are 
beneficial to each creature under its complex relations of life. 
And it may be asked what advantage, as far as we can see, 
would it be to an infusorian animalcule — to an intestinal 
worm — or even to an earth-worm, to be highly organised. 
If it were no advantage, these forms would be left, by natural 
selection, unimproved or but little improved, and might re- 
main for indefinite ages in their present lowly condition. 
And geology tells us that some of the lowest forms, as the 
infusoria and rhizopods, have remained for an enormous 
period in nearly their present state. But to suppose that most 


of the many now existing low forms have not in the least 
advanced since the first dawn of life would be extremely 
rash ; for every naturalist who has dissected some of the be- 
ings now ranked as very low in the scale, must have been 
struck with their really wondrous and beautiful organisation. 
Nearly the same remarks are applicable if we look to the 
different grades of organisation within the same great group; 
for instance, in the vertebrata, to the co-existence of mam- 
mals and fish — amongst mammalia, to the co-existence of man 
and the ornithorhynchus — amongst fishes, to the co-existence 
of the shark and the lancelet (Amphioxus), which latter fish 
in the extreme simplicity of its structure approaches the in- 
vertebrate classes. But mammals and fish hardly come into 
competition with each other ; the advancement of the whole 
class of mammals, or of certain members in this class, to the 
highest grade would not lead to their taking the place of 
fishes. Physiologists believe that the brain must be bathed 
by warm blood to be highly active, and this requires aerial 
respiration ; so that warm-blooded mammals when inhabiting 
the water lie under a disadvantage in having to come con- 
tinually to the surface to breathe. With fishes, members 
of the shark family would not tend to supplant the lancelet; 
for the lancelet, as I hear from Fritz Miiller, has as sole com- 
panion and competitor on the barren sandy shore of South 
Brazil, an anomalous annelid. The three lowest orders of 
mammals, namely, marsupials, edentata, and rodents, co-exist 
in South America in the same region with numerous monkeys, 
and probably interfere little with each other. Although or- 
ganisation, on the whole, may have advanced and be still 
advancing throughout the world, yet the scale will always 
present many degrees of perfection; for the high advance- 
ment of certain whole classes, or of certain members of each 
class, does not at all necessarily lead to the extinction of 
those groups with which they do not enter into close competi- 
tion. In some cases, as we shall hereafter see, lowly or- 
ganised forms appear to have been preserved to the present 
day, from inhabiting confined or peculiar stations, where 
they have been subjected to less severe competition, and 
where their scanty numbers have retarded the chance of fav- 
orable variations arising. 


Finally, I believe that many lowly organised forms now 
exist throughout the world, from various causes. In some 
cases variations or individual differences of a favorable na- 
ture may never have arisen for natural selection to act on and 
accumulate. In no case, probably, has time sufficed for the 
utmost possible amount of development. In some few cases 
there has been what we must call retrogression of organisa- 
tion. But the main cause lies in the fact that under very 
simple conditions of life a high organisation would be of no 
service, — possibly would be of actual disservice, as being of 
a more delicate nature, and more liable to be put out of order 
and injured. 

Looking to the first dawn of life, when all organic beings, 
as we may believe, presented the simplest structure, how, it 
has been asked, could the first steps in the advancement or 
differentiation of parts have arisen? Mr. Herbert Spencer 
would probably answer that, as soon as simple unicellular 
organism came by growth or division to be compounded of 
several cells, or became attached to any supporting surface, 
his law "that homologous units of any order became differ- 
entiated in proportion as their relations to incident forces 
became different" would come into action. But as we have 
no facts to guide us, speculation on the subject is almost use- 
less. It is, however, an error to suppose that there would 
be no struggle for existence, and, consequently, no natural 
selection, until many forms had been produced: variations 
in a single species inhabiting an isolated station might be 
beneficial, and thus the whole mass of individuals might be 
modified, or two distinct forms might arise. But, as I re- 
marked towards the close of the Introduction, no one ought 
to feel surprise at much remaining as yet unexplained on 
the origin of species, if we make due allowance for our pro- 
found ignorance on the mutual relations of the inhabitants 
of the world at the present time, and still more so during 
past ages. 


Mr. H. C. Watson thinks that I have overrated the im- 
portance of divergence of character (in which, however, he 
apparently believes), and that convergence, as it may be 


called, has likewise played a part. If two species, belonging 
to two distinct though allied genera, had both produced a 
large number of new and divergent forms, it is conceivable 
that these might approach each other so closely that they 
would have all to be classed under the same genus ; and thus 
the descendants of two distinct genera would converge into 
one. But it would in most cases be extremely rash to at- 
tribute to convergence a close and general similarity of struc- 
ture in the modified descendants of widely distinct forms. 
The shape of a crystal is determined solely by the molecular 
forces, and it is not surprising that dissimilar substances 
should sometimes assume the same form; but with organic 
beings we should bear in mind that the form of each depends 
on an infinitude of complex relations, namely on the varia- 
tions which have arisen, these being due to causes far too 
intricate to be followed out, — on the nature of the variations 
which have been preserved or selected, and this depends on 
the surrounding physical conditions, and in a still higher 
degree on the surrounding organisms with which each being 
has come into competition, — and lastly, on inheritance (in it- 
self a fluctuating element) from innumerable progenitors, 
all of which have had their forms determined through equally 
complex relations. It is incredible that the descendants of 
two organisms, which had originally differed in a marked 
manner, should ever afterwards converge so closely as to lead 
to a near approach to identity throughout their whole organ- 
isation. If this had occurred, we should meet with the same 
form, independently of genetic connection, recurring in 
widely separated geological formations ; and the balance of 
evidence is opposed to any such an admission. 

Mr. Watson has also objected that the continued action 
of natural selection, together with divergence of character, 
would tend to make an indefinite number of specific forms. 
As far as mere inorganic conditions are concerned, it seems 
probable that a sufficient numJier of species would soon 
become adapted to all considerable diversities of heat, 
moisture, &c. ; but I fully admit that the mutual relations of 
organic beings are more important ; and as the number of 
species in any country goes on increasing, the organic con- 
ditions of life must become more and more complex. Conse- 


quently there seems at first sight no limit to the amount of 
profitable diversification of structure, and therefore no limit 
to the number of species which might be produced. We do 
not know that even the most prolific area is fully stocked 
with specific forms: at the Cape of Good Hope and in Aus- 
tralia, which support such an astonishing number of species, 
many European plants have become naturalised. But geology 
shows us, that from an early part of the tertiary period the 
number of species of shells, and that from the middle part of 
this same period the number of mammals, has not greatly or 
at all increased. What then checks an indefinite increase 
in the number of species? The amount of life (I do not mean 
the number of specific forms) supported on an area must 
have a limit, depending so largely as it does on physical con- 
ditions; therefore, if an area be inhabited by very many spe- 
cies, each or nearly each species will be represented by few 
individuals; and such species will be liable to extermination 
from accidental fluctuations in the nature of the seasons 
or in the number of their enemies. The process of exter- 
mination in such cases would be rapid, whereas the production 
of new species must always be slow. Imagine the extreme 
case of as many species as individuals in England, and the 
first severe winter or very dry summer would exterminate 
thousands on thousands of species. Rare species, and each 
species will become rare if the number of species in any 
country becomes indefinitely increased, will, on the principle 
often explained, present within a given period few favorable 
variations; consequently, the process of giving birth to new 
specific forms would thus be retarded. When any species be- 
comes very rare, close interbreeding will help to exterminate 
it ; authors have thought that this comes into play in account- 
ing for the deterioration of the Aurochs in Lithuania, of Red 
Deer in Scotland, and of Bears in Norway, &c. Lastly, and 
this I am inclined to think is the most important element, a 
dominant species, which has already beaten many competitors 
in its own home, will tend to spread and supplant many others. 
Alph, de Candolle has shown that those species which spread 
widely, tend generally to spread very widely; consequently, 
they will tend to supplant and exterminate :L-everal species 
in several areas, and thus check the inordinate increase oi 


specific forms throughout the world. Dr. Hooker has re- 
cently shown that in the S.E. corner of Australia, where, 
apparently, there are many invaders from different quarters 
of the globe, the endemic Australian species have been 
greatly reduced in number. How much weight to attribute 
to these several considerations I will not pretend to say; but 
conjointly they must limit in each country the tendency to 
an indefinite augmentation of specific forms. 


If under changing conditions of life organic beings present 
individual differences in almost every part of their structure, 
and this cannot be disputed; if there be, owing to their 
geometrical rate of increase, a severe struggle for life at 
some age, season, or year, and this certainly cannot be dis- 
puted; then, considering the infinite complexity of the rela- 
tions of all organic beings to each other and to their condi- 
tions of life, causing an infinite diversity in structure, consti- 
tution, and habits, to be advantageous to them, it would be a 
most extraordinary fact if no variations had ever occurred 
useful to each being's own welfare, in the same manner as so 
many variations have occurred useful to man. But if varia- 
tions useful to any organic being ever do occur, assuredly 
individuals thus characterised will have the best chance of 
being preserved in the struggle for life; and from the strong 
principle of inheritance, these will tend to produce offspring 
similarly characterised. This principle of preservation, or the 
survival of the fittest, I have called Natural Selection. It 
leads to the improvement of each creature in relation to its 
organic and inorganic conditions of life; and consequently, 
in most cases, to what must be regarded as an advance in 
organism. Nevertheless, low and simple forms will long 
endure if well fitted for their simple conditions of life. 

Natural selection, on the principle of qualities being in- 
herited at corresponding ages, can modify the egg, seed, or 
young, as easily as the adult. Amongst many animals, sexual 
selection will have given its aid to ordinary selection, by 
assuring to the most vigorous and best adapted males the 
greatest number of offspring. Sexual selection will also give 


characters useful to the males alone, in their struggles or 
rivalry with other males ; and these characters will be trans- 
mitted to one sex or to both sexes, according to the form of 
inheritance which prevails. 

Whether natural selection has really thus acted in adapting 
the various forms of life to their several conditions and sta- 
tions, must be judged by the general tenor and balance of 
evidence given in the following chapters. But we have al- 
ready seen how it entails extinction; and how largely ex- 
tinction has acted in the world's history, geology plainly 
declares. Natural selection, also, leads to divergence of 
character; for the more organic beings diverge in structure, 
habits, and constitution, by so much the more can a large 
number be supported on the area, — of which we see proof by 
looking to the inhabitants of any small spot, and to the pro- 
ductions naturalised in foreign lands. Therefore, during the 
modification of the descendants of any one species, and dur- 
ing the incessant struggle of all species to increase in num- 
bers, the more diversified the descendants become, the better 
will be their chance of success in the battle for life. Thus 
the small differences distinguishing varieties of the same spe- 
cies, steadily tend to increase, till they equal the greater dif- 
ference? between species of the same genus, or even of 
distinct genera. 

We have seen that it is the common, the wndely-diffused 
and widely-ranging species, belonging to the larger genera 
within each class, which vary most; and these tend to trans- 
mit to their modified offspring that superiority which now 
makes them dominant in their own countries. Natural selec- 
tion, as has just been remarked, leads to divergence of 
character and to much extinction of the less improved and 
intermediate forms of life. On these principles, the nature 
of the afl^nities, and the generally well-defined distinctions 
between the innumerable organic beings in each class 
throughout the world, may be explained. It is a truly won- 
derful fact — the wonder of which we are apt to overlook 
from familiarity — that all animals and all plants through- 
out all time and space should be related to each other in 
groups, subordinate to groups, in the manner which we 
everywhere behold — namely, varieties of the same species 


most closely related, species of the same genus less closely 
and unequally related, forming sections and sub-genera, spe- 
cies of distinct genera much less closely related, and genera 
related in different degrees, forming sub-families, families, 
orders, sub-classes and classes. The several subordinate 
groups in any class cannot be ranked in a single file, but 
seem clustered round points, and these round other points, 
and so on in almost endless cycles. If. species had been in- 
dependently created, no explanation would have been pos- 
sible of this kind of classification; but it is explained through 
inheritance and the complex action of natural selection, en- 
tailing extinction and divergence of character, as we have 
seen illustrated in the diagram. 

The affinities of all the beings of the same class have some- 
times been represented by a great tree. I believe this simile 
largely speaks the truth. The green and budding twigs may 
represent existing species ; and those produced during former 
years may represent the long succession of extinct species. 
At each period of growth all the growing twigs have tried 
to branch out on all sides, and to overtop and kill the sur- 
rounding twigs and branches, in the same manner as species 
and groups of species have at all times overmastered other 
species in the great battle for life. The limbs divided into 
great branches, and these into lesser and lesser branches, 
were themselves once, when the tree was young, budding 
twigs ; and this connection of the former and present buds by 
ramifying branches may well represent the classification of 
all extinct and living species in groups subordinate to groups. 
Of the many twigs which flourished when the tree was a 
mere bush, only two or three, now grown into great branches, 
yet survive and bear the other branches ; so with the species 
which lived during long-past geological periods, very few 
have left living and modified descendants. From the first 
growth of the tree, many a limb and branch has decayed and 
dropped off; and these fallen branches of various sizes may 
represent those whole orders, families, and genera which have 
now no living representatives, and which are known to us 
only in a fossil state. As we here and there see a thin strag- 
gling branch springing from a fork low down in a tree, and 
which by some chance has been favoured and is still alive on 


its summit, so we occasionally see an animal like the Ornitho- 
rhynchus or Lepidosiren. which in some small degree con- 
nects by its affinities two large branches of life, and which 
has apparently been saved from fatal competition by having 
inhabited a protected station. As buds give rise by growth 
to fresh buds, and these, if vigorous, branch out and overtop 
on all sides many a feebler branch, so by generation I believe 
it has been with the great Tree of Life, which fills with 
its dead and broken branches the crust of the earth, and 
covers the surface with its ever-branching and beautiful 

Laws of Variation 

Effects of changed conditions — Use and disuse, combined with natural 
selection ; organs of flight and of vision — Acclimatisation — Cor- 
related variation — Compensation and economy of growth — False 
correlations — Multiple, rudimentary, and lowly organised struc- 
tures variable — Parts developed in an unusual manner are highly 
variable ; specific characters more variable than generic : second- 
ary sexual characters variable — Species of the same genus vary 
in an analogous manner — Reversions to long-lost characters — 

I HAVE hitherto sometimes spoken as if the variations — 
so common and multiform with organic beings under 
domestication, and in a lesser degree with those under 
nature — were due to chance. This, of course, is a wholly 
incorrect expression, but it serves to acknowledge plainly 
our ignorance of the cause of each particular variation. 
Some authors believe it to be as much the function of the 
reproductive system to produce individual differences, or 
slight deviations of structttre, as to make the child like its 
parents. But the fact of variations and monstrosities oc- 
curring much more frequently under domestication than 
under nature, and the greater variability of species having 
wide ranges than of those with restricted ranges, lead to the 
conclusion that variability is generally related to the condi- 
tions of life to which each species has been exposed during 
several successive generations. In the first chapter I at- 
tempted to show that changed conditions act in two ways, 
directly on the whole organisation or on certain parts alone, 
and indirectly through the reproductive system. In all cases 
there are two factors, the nature of the organism, which is 
much the most important of the two, and the nature of the 
conditions. The direct action of changed conditions leads 
to definite or indefinite results. In the latter case the organi- 



sation seems to become plastic, and we have much fluctu- 
ating variability. In the former case the nature of the 
organism is such that it yields readily, when subjected to 
certain conditions, and all, or nearly all the individuals be- 
come modified in the same way. 

It is very difficult to decide how far changed conditions, 
such as of climate, food, &c., have acted in a definite man- 
ner. There is reason to believe that in the course of time 
the effects have been greater than can be proved by clear 
evidence. But we may safely conclude that the innumer- 
able complex co-adaptations of structure, which we see 
throughout nature between various organic beings, cannot 
be attributed simply to such action. In the following cases 
the conditions seem to have produced some slight definite 
effect: E. Forbes asserts that shells at their southern limit, 
and when living in shallow water, are more brightly col- 
oured than those of the same species from further north or 
from a greater depth ; but this certainly does not always 
hold good. Mr. Gould beUeves that birds of the same species 
are more brightly coloured under a clear atmosphere, than 
when living near the coast or on islands; and Wollaston 
is convinced that residence near the sea affects the colours 
of insects. Moquin-Tandon gives a list of plants which, 
when growing near the sea-shore, have their leaves in 
some degree fleshy, though not elsewhere fleshy. These 
slightly varying organisms are interesting in as far as they 
present characters analogous to those possessed by the spe- 
cies which are confined to similar conditions. 

When a variation is of the slightest use to any being, we 
cannot tell how much to attribute to the accumulative action 
of natural selection, and how much to the definite action of 
the conditions of life. Thus, it is well known to furriers 
that animals of the same species have thicker and better 
fur the further north they live; but who can tell how much 
of this difference may be due to the warmest-clad individu- 
als having been favoured and preserved during many genera- 
tions, and how much to the action of the severe climate? 
for it would appear that climate has some direct action on 
the hair of our domestic quadrupeds. 

Instances could be given of similar varieties being pro- 


duced from the same species under external conditions of 
life as different as can well be conceived ; and, on the other 
hand, of dissimilar varieties being produced under appar- 
ently the same external conditions. Again, innumerable in- 
stances are known to every naturalist, of species keeping 
true, or not varying at all, although living under the most 
opposite climates. Such considerations as these incline me 
to lay less weight on the direct action of the surrounding 
conditions, than on a tendency to vary, due to causes of 
which we are quite ignorant. 

In some sense the conditions of life may be said, not only 
to cause variability, either directly or indirectly, but like- 
wise to include natural selection, for the conditions deter- 
mine whether this or that variety shall survive. But when 
man is the selecting agent, we clearly see that the two ele- 
ments of change are distinct ; variability is in some manner 
excited, but it is the will of man which accumulates the va- 
riations in certain directions; and it is this latter agency 
which answers to the survival of the fittest under nature. 


From the facts alluded to in the first chapter, I think there 
can be no doubt that use in our domestic animals has 
strengthened and enlarged certain parts, and disuse dimin- 
ished them; and that such modifications are inherited. 
Under free nature, we have no standard of comparison, by 
which to judge of the effects of long-continued use or dis- 
use, for we know not the parent-forms; but many animals 
possess structures which can be best explained by the effects 
of disuse. As Professor Owen has remarked, there is no 
greater anomaly in nature than a bird that cannot fly ; yet 
there are several in this state. The logger-headed duck 
of South America can only flap along the surface of the 
water, and has its wings in nearly the same condition as the 
domestic Aylcsbury-duck : it is a remarkable fact that the 
young birds, according to Mr. Cunningham, can fly, while 
the adults have lost this power. As the larger ground- 
feeding birds seldom take flight except to escape danger, 


it is probable that the nearly wingless condition of several 
birds, now inhabiting or which lately inhabited several 
oceanic islands, tenanted by no beasts of prey, has been 
caused by disuse. The ostrich indeed inhabits continents, 
and is exposed to danger from which it cannot escape by 
flight, but it can defend itself by kicking its enemies, as 
efficiently as many quadrupeds. We may believe that the 
progenitor of the ostrich genus had habits like those of 
the bustard, and that, as the size and weight of its body 
were increased during successive generations, its legs were 
used more, and its wings less, until they became incapable 
of flight. 

Kirby has remarked (and I have observed the same fact) 
that the anterior tarsi, or feet, of many male dung-feeding 
beetles are often broken off; he examined seventeen speci- 
mens in his own collection, and not one had even a relic left. 
In the Onites apelles the tarsi are so habitually lost, that 
the insect has been described as not having them. In some 
other genera they are present, but in a rudimentary condi- 
tion. In the Ateuchus or sacred beetle of the Egyptians, 
they are totally deficient. The evidence that accidental mu- 
tilations can be inherited is at present not decisive ; but the 
remarkable cases observed by Brown-Sequard in guinea- 
pigs, of the inherited effects of operations, should make us 
cautious in denying this tendency. Hence it will perhaps 
be safest to look at the entire absence of the anterior tarsi 
in Ateuchus, and their rudimentary condition in some other 
genera, not as cases of inherited mutilations, but as due to 
the effects of long-continued disuse; for as many dung- 
feeding beetles are generally found with their tarsi lost, 
this must happen early in life; therefore the tarsi cannot 
be of much importance or be much used by these insects. 

In some cases we might easily put down to disuse modifi- 
cations of structure which are wholly, or mainly, due to 
natural selection. Air. Wollaston has discovered the remark- 
able fact that 200 beetles, out of the 550 species (but more 
are now known) inhabiting Madeira, are so far defi.cient 
in wings that they cannot fly; and that, of the twenty-nine 
endemic genera, no less than twenty-three have all their spe- 
cies in this condition ! Several facts, — namely, that beetles 


in many parts of the world are frequently blown to sea and 
perish; that the beetles in ^Madeira, as observed by Mr. Wol- 
laston. lie much concealed, until the wind lulls and the sun 
shines; that the proportion of wingless beetles is larger on 
the exposed Desertas than in Madeira itself; and especially 
the extraordinary fact, so strongly insisted on by Mr. Wol- 
laston, that certain large groups of beetles, elsewhere ex- 
cessively num.erous, which absolutely require the use of their 
wings, are here almost entirely absent; — these several con- 
siderations make me believe that the wingless condition of 
so many Madeira beetles is mainly due to the action of 
natural selection, combined probably with disuse. For dur- 
ing many successive generations each individual beetle which 
flew least, either from its wings having been ever so little 
less perfectly developed or from indolent habit, will have had 
the best chance of surviving from not being blown out to 
sea; and, on the other hand, those beetles which most readily 
took to flight would oftenest have been blown to sea, and 
thus destroyed. 

The insects in Madeira which are not ground-feeders, and 
which, as certain flower- feeding coleoptera and lepidoptera, 
must habitually use their wings to gain their subsistence, 
have, as Mr. Wollaston suspects, their wings not at all re- 
duced, but even enlarged. This is quite compatible with 
the action of natural selection. For when a new insect first 
arrived on the island, the tendency of natural selection to 
enlarge or to reduce the wings, would depend on whether a 
greater number of individuals were saved by successfully 
battling with the winds, or by giving up the attempt and 
rarely or never flying. As with mariners ship-wrecked near 
a coast, it v,^ould have been better for the good swimmers if 
they had been able to swim still further, whereas it would 
have been better for the bad swimmers if they had not been 
able to swim at all and had stuck to the wreck. 

The eyes of moles and of some burrowing rodents are 
rudimentary in size, and in some cases are quite covered by 
skin and fur. This state of the eyes is probably due to 
gradual reduction from disuse, but aided perhaps by natural 
selection. In South America, a burrowing rodent, the tuco- 
tuco, or Ctenomys, is even more subterranean in its habits 


than the mole; and I was assured by a Spaniard, who had 
often caught them, that they were frequently blind. One 
which I kept alive was certainly in this condition, the cause, 
as appeared on dissection, having been inflammation of the 
nictitating membrane. As frequent inflammations of the eyes 
must be injurious to any animal, and as eyes are certainly 
not necessary to animals having subterranean habits, a re- 
duction in their size, with the adhesion of the eyelids and 
growth of fur over them, might in such case be an advan- 
tage; and if so, natural selection would aid the effects of 

It is well known that several animals, belonging to the 
most different classes, which inhabit the caves of Carniola 
and of Kentucky, are blind. In some of the crabs the foot- 
stalk for the eye remains, though the eye is gone; — the 
stand for the telescope is there, though the telescope with 
its glasses has been lost. As it is difficult to imagine that 
eyes, though useless, could be in any way injurious to ani- 
mals living in darkness, their loss may be attributed to dis- 
use. In one of the blind animals, namely, the cave-rat 
(Neotoma), two of which were captured by Professor Silli- 
raan at above half a mile distance from the mouth of the 
cave, and therefore not in the profoundest depths, the eyes 
were lustrous and of large size ; and these animals, as I am 
informed by Professor Silliman, after having been exposed 
for about a month to a graduated light, acquired a dim per- 
ception of objects. 

It is difficult to imagine conditions of life more similar 
than deep limestone caverns under a nearly similar climate; 
so that, in accordance with the old view of the blind ani- 
mals having been separately created for the American and 
European caverns, very close similarity in their organisation 
and affinities might have been expected. This is certainly 
not the case if we look at the two whole faunas; and with 
respect to the insects alone, Schiodte has remarked, "We are 
accordingly prevented from considering the entire phenome- 
non in any other light than something purely local, and the 
similarity which is exhibited in a few forms between the 
Mammoth cave (in Kentucky) and the caves in Carniola, 
otherwise than as a very plain expression of that analogy 


which subsists generally between the fauna of Europe and 
of North America." On my view we must suppose that 
American animals, having in most cases ordinary powers of 
vision, slowly migrated by successive generations from the 
outer world into the deeper and deeper recesses of the Ken- 
tucky caves, as did European animals into the caves of 
Europe. We have some evidence of this gradation of habit; 
for, as Schiodte remarks, "We accordingly look upon the 
subterranean faunas as small ramifications which have pene- 
trated into the earth from the geographically limited faunas 
of the adjacent tracts, and which, as they extended them- 
selves into darkness, have been accommodated to surround- 
ing circumstances. Animals not far remote from ordinary 
forms, prepare the transition from light to darkness. Next 
follow those that are constructed for twilight; and, last of 
all, those destined for total darkness, and whose formation is 
quite peculiar." These remarks of Schiodte's, it should be 
understood, apply not to the same, but to distinct species. 
By the time that an animal had reached, after numberless 
generations, the deepest recesses, disuse will on this view 
have more or less perfectly obliterated its eyes, and natural 
selection will often have effected other changes, such as an 
increase in the length of the antennae or palpi, as a compen- 
sation for blindness. Notwithstanding such modifications, 
we might expect still to see in the cave-animals of America, 
affinities to the other inhabitants of that continent, and in 
those of Europe to the inhabitants of the European conti- 
nent. And this is the case with some of the American cave- 
animals, as I hear from Professor Dana ; and some of the 
European cave-insects are very closely allied to those of the 
surrounding country. It would be difficult to give any ra- 
tional explanation of the affinities of the blind cave-animals 
to the other inhabitants of the two continents on the ordi- 
nary view of their independent creation. That several of 
the inhabitants of the caves of the Old and New Worlds 
should be closely related, we might expect from the well- 
known relationship of most of their other productions. As 
a blind species of Bathyscia is found in abundance on shady 
rocks far from caves, the loss of vision in the cave-species 
of this one genus has probably had no relation to its dark 


habitation ; for it is natural that an insect already deprived 
of vision should readily become adapted to dark caverns. 
Another blind genus (Anophthalmus) offers this remark- 
able peculiarity, that the species, as Mr. Murray observes, 
have not as yet been found anywhere except in caves, yet 
those which inhabit the several caves of Europe and America 
are distinct; but it is possible that the progenitors of these 
several species, whilst they were furnished with eyes, may 
formerly have ranged over both continents, and then have 
become extinct, excepting in their present secluded abodes. 
Far from feeling surprise that some of the cave-animals 
should be very anomalous, as Agassiz has remarked in re- 
gard to the blind fish, the Amblyopsis, and as is the case 
with the blind Proteus with reference to the reptiles of 
Europe, I am only surprised that more wrecks of ancient 
life have not been preserved, owing to the less severe com- 
petition to which the scanty inhabitants of these dark abodes 
will have been exposed. 


Habit is hereditary with plants, as in the period of flower- 
ing, in the time of sleep, in the amount of rain requisite for 
seeds to germinate, &c., and this leads me to say a few 
words on acclimatisation. As it is extremely common for 
distinct species belonging to the same genus to inhabit hot 
and cold countries, if it be true that all the species of the 
same genus are descended from a single parent-form, accli- 
matisation must be readily effected during a long course of 
descent It is notorious that each species is adapted to the 
climate of its own home: species from an arctic or even from 
a temperate region cannot endure a tropical climate, or con- 
versely. So again, many succulent plants cannot endure a 
damp climate. But the degree of adaptation of species to 
the climates under which they live is often overrated. We 
may infer this from our frequent inability to predict whether 
or not an imported plant will endure our climate, and from 
the number of plants and animals brought from different 
countries which are here perfectly healthy. We have rea- 
son to believe that species in a state of nature are closely 


limited in their ranges by the competition of other organic 
beings quite as much as, or more than, by adaptation to par- 
ticular cHmates. But whether or not this adaptation is in 
most cases very close, we have evidence with some few 
plants, of their becoming, to a certain extent, naturally 
habituated to different temperatures; that is, they become 
acclimatised: thus the pines and rhododendrons, raised from 
seed collected by Dr. Hooker from the same species grow- 
ing at dififerent heights on the Himalaya, were found to pos- 
sess in this country different constitutional powers of re- 
sisting cold. Mr. Thwaites informs me that he has observed 
similar facts in Ceylon ; analogous observations have been 
made by Mr. H. C. Watson on European species of plants 
brought from the Azores to England ; and I could give other 
cases. In regard to animals, several authentic instances 
could be adduced of species having largely extended, within 
historical times, their range from warmer to cooler lati- 
tudes, and conversely ; but we do not positively know that 
these animals were strictly adapted to their native climate, 
though in all ordinary cases we assume such to be the case ; 
nor do we know that they have subsequently become specially 
acclimatised to their new homes, so as to be better fitted for 
them than they were at first. 

As we may infer that our domestic animals were originally 
chosen by uncivilised man because they were useful and be- 
cause they bred readily under confinement, and not because 
they were subsequently found capable of far-extended trans- 
portation, the common and extraordinary capacity in our 
domestic animals of not only withstanding the most different 
climates, but of being perfectly fertile (a far severer test) 
under them, may be used as an argument that a large pro- 
portion of other animals now in a state of nature could 
easily be brought to bear widely different climates. We 
must not, however, push the foregoing argument too far, 
on account of the probable origin of some of our domestic 
animals from several wild stocks; the blood, for instance, 
of a tropical and arctic wolf may perhaps be mingled in our 
domestic breeds. The rat and mouse cannot be considered as 
domestic animals, but they have been transported by man to 
many parts of the world, and now have a far wider range 


than any other rodent; for they live under the cold climate 
of Faroe in the north and of the Falklands in the south, 
and on many an island in the torrid zones. Hence adap- 
tation to any special climate may be looked at as a quality 
readily grafted on an innate wide flexibility of constitution, 
common to most animals. On this view, the capacity of 
enduring the most different climates by man himself and 
by his domestic animals, and the fact of the extinct elephant 
and rhinoceros having formerly endured a glacial climate, 
whereas the living species are now all tropical or sub-tropical 
in their habits, ought not to be looked at as anomalies, but 
as examples of a very common flexibility of constitution, 
brought, under peculiar circumstances, into action. 

How much of the acclimatisation of species to any pecu- 
liar climate is due to mere habit, and how much to the 
natural selection of varieties having different innate consti- 
tutions, and how much to both means combined, is an ob- 
scure question. That habit or custom has some influence, I 
must believe, both from analogy and from the incessant ad- 
vice given in agricultural works, even in the ancient Ency- 
clopaedias of China, to be very cautious in transporting ani- 
mals from one district to another. And as it is not likely 
that man should have succeeded in selecting so many breeds 
and sub-breeds with constitutions specially fitted for their 
own districts, the result must, I think, be due to habit. On 
the other hand, natural selection would inevitably tend to 
preserve those individuals which were born with consti- 
tutions best adapted to any country which they inhabited. 
In treatises on many kinds of cultivated plants, certain 
varieties are said to withstand certain climates better than 
others; this is strikingly shown in works on fruit-trees pub- 
lished in the United States, in which certain varieties are 
habitually recommended for the northern and others for the 
southern States ; and as most of these varieties are of recent 
origin, they cannot owe their constitutional differences to 
habit. The case of the Jerusalem artichoke, which is never 
propagated in England by seed, and of which consequently 
new varieties have not been produced, has even been ad- 
vanced, as proving that acclimatisation cannot be effected, 
for it is now as tender as ever it was ! The case, also, of the 


kidney-bean has been often cited for a similar purpose, and 
with much greater weight; but until someone will sow,' dur- 
ing a score of generations, his kidney-beans so early that a 
very large proportion are destroyed by frost, and then collect 
seed from the few survivors, with care to prevent accidental 
crosses, and then again get seed from these seedlings, with 
the same precautions, the experiment cannot be said to have 
been tried. Nor let it be supposed that differences in the con- 
stitution of seedling kidney-beans never appear, for an ac- 
count has been published how much more hardy some seed- 
lings are than others; and of this fact I have myself ob- 
served striking instances. 

On the whole, we may conclude that habit, or use and 
disuse, have, in some cases, played a considerable part in the 
modification of the constitution and structure ; but that the 
effects have often been largely combined with, and some- 
times overmastered by, the natural selection of innate 


I mean by this expression that the whole organisation is 
so tied together during its growth and development, that 
when slight variations in any one part occur, and are accu- 
mulated through natural selection, other parts become modi- 
fied. This is a very important subject, most imperfectly 
understood, and no doubt wholly dift'ercnt classes of facts 
may be here easily confounded together. We shall presently 
see that simple inheritance often gives the false appearance 
of correlation. One of the most obvious real cases is, that 
variations of structure arising in the young or larvre nat- 
urally tend to affect the structure of the mature animal. 
The several parts of the body which are homologous, and 
which, at an early embryonic period, are identical in struc-_ 
ture, and which are necessarily exposed to similar condi- 
tions, seem eminently liable to vary in a like manner: we see 
this in the right and left sides of the body varying in the 
same manner; in the front and hind legs, and even in the 
jaws and limbs, varying together, for the lower jaw is be- 
lieved by some anatomists to be homologous with the limbs. 
These tendencies, I do not doubt, may be mastered more or 


less completely by natural selection; thus a family of stags 
once existed with an antler only on one side; and if this 
had been of any great use to the breed, it might probably 
have been rendered permanent by selection. 

Homologous parts, as has been remarked by some authors, 
tend to cohere; this is often seen in monstrous plants: and 
nothing is more common than the union of homologous 
parts in normal structures, as in the union of the petals into 
a tube. Hard parts seem to affect the form of adjoining 
soft parts; it is believed by some authors that with birds 
the diversity in the shape of the pelvis causes the remark- 
able diversity in the shape of their kidneys. Others believe 
that the shape of the pelvis in the human mother influences 
by pressure the shape of the head of the child. In snakes, 
according to Schlegel, the form of the body and the manner 
of swallowing determine the position and form of several 
of the most important viscera. 

The nature of the bond is frequently quite obscure. M. 
Is. Geoffroy St. Hilaire has forcibly remarked, that certain 
malconformations frequently, and that others rarely, co- 
exist, without our being able to assign any reason. What 
can be more singular than the relation in cats between com- 
plete whiteness and blue eyes with deafness, or between the 
tortoise-shell colour and the female sex; or in pigeons be- 
tween their feathered feet and skin betwixt the outer toes, 
or between the presence of more or less down on the young 
pigeon when first hatched, with the future colour of its 
plumage; or, again, the relation between the hair and teeth 
in the naked Turkish dog, though here no doubt homology 
comes into play? With respect to this latter case of corre- 
lation, I think it can hardly be accidental, that the two orders 
of mammals which are most abnormal in their dermal cov- 
erings, viz., Cetacea (whales) and Edentata (armadilloes, 
scaly ant-eaters, &c.), are likewise on the whole the most 
abnormal in their teeth ; but there are so many exceptions to 
this rule, as Mr. Mivart has remarked, that it has little 


I know of no case better adapted to show the importance 
of the laws of correlation and variation, independently of 
utility and therefore of natural selection, than that of the 


difference between the outer and inner flowers in some Com- 
positous and Umbelliferous plants. Every one is familiar 
with the difference between the ray and central florets of, 
for instance, the daisy, and this dift'erence is often accom- 
panied with the partial or complete abortion of the repro- 
ductive organs. But in some of these plants, the seeds also 
differ in shape and sculpture. These differences have some- 
times been attributed to the pressure of the involucra on 
the florets, or to their mutual pressure, and the shape of the 
seeds in the ray-florets of some Compositae countenances this 
idea; but with the Umbelliferae, it is by no means, as Dr. 
Hooker informs me, the species with the densest heads which 
most frequently differ in their inner and outer flowers. It 
might have been thought that the development of the ray- 
petals by drawing nourishment from the reproductive 
organs causes their abortion; but this can hardly be the sole 
cause, for in some Compositze the seeds of the outer and 
inner florets differ, without any difference in the corolla. 
Possibly these several differences may be connected with 
the different flow of nutriment towards the central and 
external flowers : we know, at least, that with irregular 
flowers, those nearest to the axis are most subject to peloria, 
that is to become abnormally symmetrical. I may add, as 
an instance of this fact, and as a striking case of correla- 
tion, that in many pelargoniums, the two upper petals in the 
central flower of the truss often lose their patches of darker 
colour; and when this occurs, the adherent nectary is quite 
aborted; the central flower thus becoming peloric or regular. 
When the colour is absent from only one of the two upper 
petals, the nectary is not quite aborted but is much shortened. 
With respect to the development of the corolla, Sprengel's 
idea that the ray-florets serve to attract insects, whose 
agency is highly advantageous or necessary for the fertili- 
sation of these plants, is highly probable; and if so. natural 
selection may have come into play. But with respect to the 
seeds, it seems impossible that their differences in shape, 
which are not always correlated with any difference in the 
corolla, can be in any way beneficial: yet in the UmbcUifera; 
these differences are of such apparent importance — the seeds 
being sometimes orthospermous in the exterior flowers and 


coelospermous in the central flowers, — that the elder De 
Candolle founded his main divisions in the order on such 
characters. Hence modifications of structure, viewed by 
systematists as of high value, may be wholly due to the laws 
of variation and correlation, without being, as far as we 
can judge, of the slightest service to the species. 

We may often falsely attribute to correlated variation 
structures which are common to whole groups of species, 
and which in truth are simply due to inheritance; for an 
ancient progenitor may have acquired through natural selec- 
tion some one modification in structure, and, after thousands 
of generations, some other and independent modification; 
and these two modifications, having been transmitted to a 
whole group of descendants with diverse habits, would nat- 
urally be thought to be in some necessary manner correlated. 
Some other correlations are apparently due to the manner 
in which natural selection can alone act. For instance, Alph. 
de Candolle has remarked that winged seeds are never found 
in fruits which do not open; I should explain this rule by 
the impossibility of seeds gradually becoming winged through 
natural selection, unless the capsules were open for in this 
case alone could the seeds, which were a little better adapted 
to be wafted by the wind, gain an advantage over others 
less well fitted for wide dispersal. 


The elder Geoffroy and Goethe propounded, at about the 
same time, their law of compensation or balancement of 
growth; or, as Goethe expressed it, "in order to spend on 
one side, nature is forced to economise on the other side." 
I think this holds true to a certain extent with our domestic 
productions: if nourishment flows to one part or organ in 
excess, it rarely flows, at least in excess, to another part; 
thus it is difficult to get a cow to give much milk and to fat- 
ten readily. The same varieties of the cabbage do not yield 
abundant and nutritious foliage and a copious supply of oil- 
bearing seeds. When the seeds in our fruits become atro- 
phied, the fruit itself gains largely in size and quality. In 
our poultry, a large tuft of feathers on the head is gener- 


ally accompanied by a diminished comb and a large beard 
by diminished wattles. With species in a state of nature it 
can hardly be maintained that the law is of universal appli- 
cation ; but many good observers, more especially botanists, 
believe in its truth. I will not, however, here give any in- 
stances, for I sec hardly any way of distinguishing between 
the effects, on the one hand, of a part being largely devel- 
oped through natural selection and another and adjoining 
part being reduced by this same process or by disuse, and, 
on the other hand, the actual withdrawal of nutriment from 
one part owing to the excess of growth in another and ad- 
joining part. 

I suspect, also, that some of the cases of compensation 
which have been advanced, and likewise some other facts, 
may be merged under a more general principle, namely, that 
natural selection is continually trying to economise every 
part of the organisation. If under changed conditions of 
life a structure, before useful, becomes less useful, its dim- 
inution will be favoured, for it will profit the individual not 
to have its nutriment wasted in building up an useless struc- 
ture. I can thus only understand a fact with which I was 
much struck when examining cirripedes, and of which many 
analogous instances could be given: namely, that when a 
cirripede is parasitic within another cirripede and is thus 
protected, it loses more or less completely its own shell or 
carapace. This is the case with the male Ibla, and in a truly 
extraordinary manner with the Proteolepas: for the cara- 
pace in all other cirripedes consists of the three highly im- 
portant anterior segments of the head enormously developed, 
and furnished with great nerves and muscles; but in the 
parasitic and protected Proteolepas, the whole anterior part 
of the head is reduced to the merest rudiment attached to 
the bases of the prehensile antennc-e. Now the saving of a 
large and complex structure, when rendered superfluous, 
would be a decided advantage to each successive individual 
of the species; for in the struggle for life to which every 
animal is exposed, each would have a better chance of sup- 
porting itself, by less nutriment being wasted. 

Thus, as I believe, natural selection will tend in the long 
run to reduce any part of the organisation, as soon as it be- 


comes, through changed habits, superfluous, without by any 
means causing some other part to be largely developed in a 
corresponding degree. And, conversely, that natural selec- 
tion may perfectly well succeed in largely developing an 
organ without requiring as a necessary compensation the 
reduction of some adjoining part. 


It seems to be a rule, as remarked by Is. Geoffroy St. 
Hilaire, both with varieties and species, that when any part 
or organ is repeated many times in the same individual (as 
the vertebrae in snakes, and the stamens in polyandrous flow- 
ers) the number is variable ; whereas the same part or organ, 
when it occurs in lesser numbers, is constant. The same 
author as well as some botanists have further remarked that 
multiple parts are extremely liable to vary in structure. As 
"vegetative repetition," to use Prof. Owen's expression, is a 
sign of low organisation, the foregoing statements accord 
with the common opinion of naturalists, that beings which 
stand low in the scale of nature are more variable than those 
which are higher. I presume that lowness here means that 
the several parts of the organisation have been but little 
specialised for particular functions; and as long as the same 
part has to perform diversified work, we can perhaps see 
why it should remain variable, that is, why natural selection 
should not have preserved or rejected each little deviation 
of form so carefully as when the part has to serve for some 
one special purpose. In the same way that a knife which 
has to cut all sorts of things may be of almost any shape; 
whilst a tool for some particular purpose must be of some 
particular shape. Natural selection, it should never be for- 
gotten, can act solely through and for the advantage of each 

Rudimentary parts, as it is generally admitted, are apt to be 
highly variable. We shall have to recur to this subject; and 
I will here only add that their variability seems to result from 
their uselessness, and consequently from natural selection 
having had no power to check deviations in their structure. 






Several years ago I was much struck by a remark, to the 
above effect, made by Mr. Waterhouse. Professor Owen, 
also, seems to have come to a nearly similar conclusion. It 
is hopeless to attempt to convince any one of the truth of 
the above proposition without giving the long array of facts 
which I have collected, and which cannot possibly be here 
introduced. I can only state my conviction that it is a rule 
of high generality. I am aware of several causes of error, 
but I hope that I have made due allowance for them. 
It should be understood that the rule by no means applies to 
any part, however unusually developed, unless it be unusu- 
ally developed in one species or in a few species in compari- 
son with the same part in many closely allied species. Thus, 
the wing of a bat is a most abnormal structure in the class 
of mammals, but the rule would not apply here, because the 
whole group of bats possesses wings; it would apply only if 
some one species had wings developed in a remarkable man- 
ner in comparison with the other species of the same genus. 
The rule applies very strongly in the case of secondary sex- 
ual characters, when displayed in any unusual manner. The 
term, secondary sexual characters, used by Hunter, relates 
to characters which are attached to one sex, but are not 
directly connected with the act of reproduction. The rule 
applies to males and females ; but more rarely to the females, 
as they seldom offer remarkable secondary sexual charac- 
ters. The rule being so plainly applicable in the case of sec- 
ondary sexual characters, may be due to the great variability 
of these characters, whether or not displayed in any unusual 
manner — of which fact I think there can be little doubt. But 
that our rule is not confined to secondary sexual characters 
is clearly shown in the case of hermaphrodite cirripedes : I 
particularly attended to Mr. Waterhouse's remark, whilst 
investigating this Order, and I am fully convinced that the 
rule almost always holds good. I shall, in a future work, 
give a list of all the more remarkable cases; I will here give 



only one, as it illustrates the rule in its largest application. 
The opercular valves of sessile cirripedes (rock barnacles) 
are, in every sense of the word, very important structures, 
and they differ extremely little even in distinct genera; but 
in the several species of one genus, Pyrgoma, these valves 
present a marvellous amount of diversification ; the homolo- 
gous valves in the different species being sometimes wholly 
Mnlike in shape; and the amount of variation in the indi- 
viduals of the same species is so great, that it is no exag- 
geration to state that the varieties of the same species differ 
more from each other in the characters derived from these 
important organs, than do the species belonging to other 
distinct genera. 

As with birds the individuals of the same species, inhabit- 
ing the same coimtry, vary extremely little, I have particu- 
larly attended to them; and the rule certainly seems to hold 
good in this class. I cannot make out that it applies to plants, 
and this would have seriously shaken my belief in its truth, 
had not the great variability in plants made it particularly 
difficult to compare their relative degrees of variability. 

When we see any part or organ developed in a remarkable 
degree or manner in a species, the fair presumption is that 
it is of high importance to that species : nevertheless it is in 
this case eminently liable to variation. Why should this be 
so? On the view that each species has been independently 
created, with all its parts as we now see them, I can see no 
explanation. But on the view that groups of species are de- 
scended from some other species, and have been modified 
through natural selection, I think we can obtain some light. 
First let me make some preliminary remarks. If, in our 
domestic animals, any part or the whole animal be neglected, 
and no selection be applied, that part (for instance, the comb 
in the Dorking fowl) or the whole breed will cease to have 
a uniform character: and the breed may be said to be degen- 
erating. In rudimentary organs, and in those which have 
been but little specialised for any particular purpose, and 
perhaps in polymorphic groups, we see a nearly parallel case ; 
for in such cases natural selection either has not or cannot 
have come into full play, and thus the organisation is left in 
a fluctuating condition. But what here more particularly 


concerns tis is, that those points in our domestic animals, 
which at the present time are undergoing rapid change by 
continued selection, are also eminently liable to variation. 
Look at the individuals of the same breed of the pigeon, and 
see what a prodigious amount of difference there is in the 
beaks of tumblers, in the beaks and wattle of carriers, in the 
carriage and tail of fantails, &c., these being the points now 
mainly attended to by English fanciers. Even in the same 
sub-breed, as in that of the short-faced tumbler, it is notori- 
ously difficult to breed nearly perfect birds, many departing 
widely from the standard. There may truly be said to be a 
constant struggle going on between, on the one hand, the 
tendency to reversion to a less perfect state, as well as an 
innate tendency to new variations, and, on the other hand, 
the power of steady selection to keep the breed true. In the 
long run selection gains the day, and we do not expect to 
fail so completely as to breed a bird as coarse as a common 
tumbler pigeon from a good short-faced strain. But as long as 
selection is rapidly going on, much variability in the parts 
undergoing modification may always be expected. 

Now let us turn to nature. When a part has been devel- 
oped in an extraordinary manner in any one species, com- 
pared with the other species of the same genus, we may con- 
clude that this part has undergone an extraordinary amount 
of modification since the period when the several species 
branched off from the common progenitor of the genus. This 
period will seldom be remote in any extreme degree, as species 
rarely endure for more than one geological period. An extra- 
ordinary amount of modification implies an unusually large 
and long-continued amount of variability, which has con- 
tinually been accumulated by natural selection for the benefit 
of the species. But as the variability of the extraordinarily 
developed part or organ has been so great and long-continued 
within a period not excessively remote, we might, as a gen- 
eral rule, still expect to find more variability in such parts 
than in other parts of the organisation which have remained 
for a much longer period nearly constant. And this, I am 
convinced, is the case. That the struggle between natural 
selection on the one hand, and the tendency to reversion and 
variability on the other hand, will in the course of time 


cease; and that the most abnormally developed organs may 
be made constant, I see no reason to doubt. Hence, when 
an organ, however abnormal it may be, has been transmitted 
in approximately the same condition to many modified de- 
scendants, as in the case of the wing of the bat, it must have 
existed, according to our theory, for an immense period in 
nearly the same state; and thus it has come not to be more 
variable than any other structure. It is only in those cases 
in which the modification has been comparatively recent and 
extraordinarily great that we ought to find the generative 
variability, as it may be called, still present in a high degree. 
For in this case the variability will seldom as yet have been 
fixed by the continued selection of the individuals varying 
in the required manner and degree, and by the continued 
rejection of those tending to revert to a former and less- 
modified condition. 



The principle discussed under the last heading may be 
applied to our present subject. It is notorious that specific 
characters are more variable than generic. To explain by a 
simple example what is meant: if in a large genus of plants 
some species had blue flowers and some had red, the colour 
would be only a specific character, and no one would be sur- 
prised at one of the blue species varying into red, or con- 
versely; but if all the species had blue flowers, the colour 
would become a generic character, and its variation would 
be a more unusual circumstance. I have chosen this exam- 
ple because the explanation which most naturalists would 
advance is not here applicable, namely, that specific charac- 
ters are more variable than generic, because they are taken 
from parts of less physiological importance than those com- 
monly used for classing genera. I believe this explanation 
is partly, yet only indirectly, true; I shall, however, have to 
return to this point in the chapter on Classification. It would 
be almost superfluous to adduce evidence in support of the 
statement, that ordinary specific characters are more variable 
than generic; but with respect to important characters, I 


have repeatedly noticed in works on natural history, that 
when an author remarks with surprise that some important 
organ or part, wliich is generally very constant throughout 
a large group of species, differs considerably in closely 
allied species, it is often variable in the individuals of the 
same species. And this fact shows that a character, which 
is generally of generic value, when it sinks in value and 
becomes only of specific value, often becomes variable, 
though its physiological importance may remain the same. 
Something of the same kind applies to monstrosities: at 
least Is. Geoffroy St. Hilaire apparently entertains no doubt, 
that the more an organ normally differs in the different spe- 
cies of the same group, the more subject it is to anomalies 
in the individuals. 

On the ordinary view of each species having been inde- 
pendently created, why should that part of the structure, 
which differs from the same part in other independently 
created species of the same genus, be more variable than 
those parts which are closely alike in the several species? 
I do not see that any explanation can be given. But on the 
view that species are only strongly marked and fixed varie- 
ties, we might expect often to find them still continuing to 
vary in those parts of their structure which have varied 
within a moderately recent period, and which have thus 
come to differ. Or to state the case in another manner : — 
the points in which all the species of a genus resemble each 
other, and in which they differ from allied genera, are called 
generic characters; and these characters may be attributed 
to inheritance from a common progenitor, for it can rarely 
have happened that natural selection will have modified sev- 
eral distinct species, fitted to more or less widely different 
habits, in exactly the same manner : and as these so-called 
generic characters have been inherited from before the 
period when the several species first branched off from their 
common progenitor, and subsequently have not varied or 
come to differ in any degree, or only in a slight degree, it is 
not probable that they should vary at the present day. On 
the other hand, the points in which species differ from other 
species of the same genus are called specific characters; and 
as these specific characters have varied and come to differ 


since the period when the species branched off from a com- 
mon progenitor, it is probable that they should still often be 
in some degree variable, — at least more variable than those 
parts of the organisation which have for a very long period 
remained constant. 

Secondary Sexual Characters Variable. — I think it will be 
admitted by naturalists, without my entering on details, that 
secondary sexual characters are highly variable. It will also 
be admitted that species of the same group differ from each 
other more widely in their secondary sexual characters, than 
in other parts of their organisation: compare, for instance, 
the amount of difference between the males of gallinaceous 
birds, in which secondary sexual characters are strongly dis- 
played, with the amount of difference between the females. 
The cause of the original variability of these characters is 
not manifest; but we can see why they should not have been 
rendered as constant and uniform as others, for they are 
accumulated by sexual selection, which is less rigid in its ac- 
tion than ordinary selection, as it does not entail death, but 
only gives fewer offspring to the less favoured males. What- 
ever the cause may be of the variability of secondary sexual 
characters, as they are highly variable, sexual selection will 
have had a wide scope for action, and may thus have suc- 
ceeded in giving to the species of the same group a greater 
amount of difference in these than in other respects. 

It is a remarkable fact, that the secondary differences be- 
tween the two sexes of the same species are generally dis- 
played in the very same parts of the organisation in which 
the species of the same genus differ from each other. Of 
this fact I will give in illustration the two first instances 
which happen to stand on my list; and as the differences in 
these cases are of a very unusual nature, the relation can 
hardly be accidental. The same number of joints in the tarsi 
is a character common to very large groups of beetles, but 
in the Engidas, as Westwood has remarked, the number varies 
greatly; and the number likewise differs in the two sexes of 
the same species. Again in the fossorial hymenoptera, the 
neuration of the wings is a character of the highest impor- 
tance, because common to large groups ; but in certain genera 
the neuration differs in the different species, and likewise in 


the two sexes of the same species. Sir J. Lubbock has re- 
cently remarked, that several minute crustaceans offer ex- 
cellent illustrations of this law. "In Pontella, for instance, 
the sexual characters are afforded mainly by the anterior 
antennas and by the fifth pair of legs: the specific differences 
also are principally given by these organs." This relation 
has a clear meaning on my view: I look at all the species 
of the same genus as having as certainly descended from 
a common progenitor, as have the two sexes of any one spe- 
cies. Consequently, whatever part of the structure oi the 
common progenitor, or of its early descendants, became vari- 
ble, variations of this part would, it is highly probable, be 
taken advantage of by natural and sexual selection, in order 
to fit the several species to their several places in the econ- 
omy of nature, and likewise to fit the two sexes of the same 
species to each other, or to fit the males to struggle with 
other males for the possession of the females. 

Finally, then, I conclude that the greater variability of 
specific characters, or those which distinguish species from 
species, than of generic characters, or those which are pos- 
sessed by all the species; — that the frequent extreme varia- 
bility of any part which is developed in a species in an extra- 
ordinary manner in comparison with the same part in its 
congeners; and the slight degree of variability in a part, 
however extraordinarily it may be developed, if it be com- 
mon to a whole group of species; — that the great variability 
of secondary sexual characters, and their great difference in 
closely allied species; — that secondary sexual and ordinary 
specific differences are generally displayed in the same parts 
of the organisation, — are all principles closely connected to- 
gether. All being mainly due to the species of the same 
group being the descendants of a common progenitor, from 
whom they have inherited much in common, — to parts which 
have recently and largely varied being more likely still to go 
on varying than parts which have long been inherited and 
have not varied — to natural selection having more or less 
completely, according to the lapse of time, overmastered the 
tendency to reversion and to further variability, — to sexual 
selection being less rigid than ordinary selection, — and to 


variations in the same parts having been accumulated by 
natural and sexual selection, and having been thus adapted 
for secondary sexual, and for ordinary purposes. 

Distinct Species present analogous Variations, so that a 
Variety of one Species often assumes a Character proper to 
an allied Species, or reverts to some of the Characters of an 
early Progenitor. — These propositions will be most readily 
understood by looking to our domestic races. The most dis- 
tinct breeds of the pigeon, in countries widely apart, present 
sub-varieties with reversed feathers on the head, and with 
feathers on the feet, — characters not possessed by the abo- 
riginal rock-pigeon; these then are analogous variations in 
two or more distinct races. The frequent presence of four- 
teen or even sixteen tail-feathers in the pouter may be con- 
sidered as a variation representing the normal structure of 
another race, the fantail. I presume that no one will doubt 
that all such analogous variations are due to the several 
races of the pigeon having inherited from a common parent 
the same constitution and tendency to variation, when acted 
on by similar unknown influences. In the vegetable king- 
dom we have a case of analogous variation, in the enlarged 
stems, or as commonly called roots, of the Swedish turnip 
and Ruta baga, plants which several botanists rank as varie- 
ties produced by cultivation from a common parent: if this 
be not so, the case will then be one of analogous variation 
in two so-called distinct species; and to these a third may be 
added, namely, the common turnip. According to the ordi- 
nary view of each species having been independently created, 
we should have to attribute this similarity in the enlarged 
stems of these three plants, not to the vera causa of com- 
munity of descent, and a consequent tendency to vary in a 
like manner, but to three separated yet closely related acts 
of creation. Many similar cases of analogous variation have 
been observed by Naudin in the great gourd-family, and by 
various authors in our cereals. Similar cases occurring with 
insects under natural conditions have lately been discussed 
with much ability by Mr. Walsh, who has grouped them 
under his law of Equable Variability. 

With pigeons, however, we have another case, namely, 
the occasional appearance in all the breeds, of slaty-blue 


birds with two black bars on the wings, white loins, a bar 
at the end of the tail, with the outer feathers externally 
edged near their basis with white. As all these marks are 
characteristic of the parent rock-pigeon, I presume that no 
one will doubt that this is a case of reversion, and not of a 
new yet analogous variation appearing in the several breeds. 
We may, I think, confidently come to this conclusion, be- 
cause, as we have seen, these coloured marks are eminently 
liable to appear in the crossed offspring of two distinct and 
differently coloured breeds; and in this case there is nothing 
in the external conditions of life to cause the reappearance 
of the slaty-blue, with the several marks, beyond the influ- 
ence of the mere act of crossing on the laws of inheritance. 
No doubt it is a very surprising fact that characters should 
reappear after having been lost for many, probably for hun- 
dreds of generations. But when a breed has been crossed 
only once by some other breed, the offspring occasionally 
show for many generations a tendency to revert in character 
to the foreign breed — some say, for a dozen or even a score 
of generations. After twelve generations, the proportion of 
blood, to use a common expression, from one ancestor, is 
only I in 2048; and yet, as we see, it is generally believed 
that a tendency to reversion is retained by this remnant of 
foreign blood. In a breed which has not been crossed but 
in which both parents have lost some character which their 
progenitor possessed, the tendency, whether strong or weak, 
to reproduce the lost character might, as was formerly re- 
marked, for all that we can see to the contrary, be trans- 
mitted for almost any number of generations. When a 
character which has been lost in a breed, reappears after a 
great number of generations, the most probable hypothesis 
is, not that one individual suddenly takes after an ancestor 
removed by some hundred generations, but that in each suc- 
cessive generation the character in question has been lying 
latent, and at last, under unkno\\ai favourable conditions, is 
developed. With the barb-pigeon, for instance, which very 
rarely produces a blue bird, it is probable that there is a 
latent tendency in each generation to produce blue plumage. 
The abstract improbability of such a tendency being trans- 
mitted through a vast number of generations, is not greater 


than that of quite useless or rudimentary organs being simi- 
larly transmitted. A mere tendency to produce a rudiment 
is indeed sometimes thus inherited. 

As all the species of the same genus are supposed to be 
descended from a common progenitor, it might be expected 
that they would occasionally vary in an analogous manner; 
so that the varieties of two or more species would resemble 
each other, or that a variety of one species would resemble 
in certain characters another and distinct species, — this other 
species being, according to our view, only a well-marked and 
permanent variety. But characters exclusively due to analo- 
gous variation would probably be of an unimportant nature, 
for the preservation of all functionally important characters 
will have been determined through natural selection, in ac- 
cordance with the different habits of the species. It might 
further be expected that the species of the same genus would 
occasionally exhibit reversions to long lost characters. As, 
however, we do not know the common ancestor of any 
natural group, we cannot distinguish between revisionary 
and analogous characters. If, for instance, we did not know 
that the parent rock-pigeon was not feather-footed or turn- 
crowned, we could not have told, whether such characters in 
our domestic breeds were reversions or only analogous varia- 
tions; but we might have inferred that the blue colour was a 
case of reversion from the number of the markings, which 
are correlated with this tint, and which would not probably 
have all appeared together from simple variation. More 
especially we might have inferred this, from the blue colour 
and the several marks so often appearing when differently 
coloured breeds are crossed. Hence, although under nature it 
must generally be left doubtful, what cases are reversions to 
formerly existing characters, and what are new but analo- 
gous variations, yet we ought, on our theory, sometimes to 
find the varying offspring of a species assuming characters 
which are already present in other members of the same 
group. And this undoubtedly is the case. 

The difficulty in distinguishing variable species is largely 
due to the varieties mocking, as it were, other species of the 
same genus. A considerable catalogue, also, could be given 
of forms intermediate between two other forms, which them- 


selves can only doubtfully be ranked as species; and this 
shows, unless all these closely allied forms be considered as 
independently created species, that they have in varying as- 
sumed some of the characters of the others. But the best 
evidence of analogous variations is afforded by parts or 
organs which are generally constant in character, but which 
occasionally vary so as to resemble, in some degree, the 
same part or organ in all species. I have collected a long 
list of such cases; but here, as before, I lie under the great 
disadvantage of not being able to give them. I can only re- 
peat that such cases certainly occur, and seem to me very re- 

I will, however, give one curious and complex case, not 
indeed as affecting any important character, but from occur- 
ring in several species of the same genus, partly under 
domestication and partly under nature. It is a case almost 
certainly of reversion. The ass sometimes has very distinct 
transverse bars on its legs, like those on the legs of the 
zebra: it has been asserted that these are plainest in the foal, 
and, from inquiries which I have made, I believe this to be 
true. The stripe on the shoulder is sometimes double, and is 
very variable in length and outline. A white ass, but not an 
albino, has been described without either spinal or shoulder 
stripe : and these stripes are sometimes very obscure, or actu- 
ally quite lost, in dark-coloured asses. The koulan of Pallas 
is said to have been seen with a double shoulder-stripe. Mr. 
Blyth has seen a specimen of the hemionus with a distinct 
shoulder-stripe, though it properly has none ; and I have been 
informed by Colonel Poole that the foals of this species are 
generally striped on the legs, and faintly on the shoulder. 
The quagga, though so plainly barred like a zebra over the 
body, is without bars on the legs; but Dr. Gray has figured 
one specimen with very distinct zebra-like bars on the hocks. 

With respect to the horse, I have collected cases in Eng- 
land of the spinal stripe in horses of the most distinct breeds, 
and of all colours: transverse bars on the legs are not rare 
in duns, mouse-duns, and in one instance in a chestnut ; a 
faint shoulder-stripe may sometimes be seen in duns, and T 
have seen a trace in a bay horse. My son made a careful 
examination and sketch for me of a dun Belgian cart-horse 


with a double stripe on each shoulder and with leg-stripes; 
I have myself seen a dun Devonshire pony, and a small dun 
Welsh pony has been carefully described to me, both with 
three parallel stripes on each shoulder. 

In the north-west part of India the Kattywar breed of 
horses is so generally striped, that, as I hear from Colonel 
Poole, who examined this breed for the Indian Government, 
a horse without stripes is not considered as purely-bred. 
The spine is always striped; the legs are generally barred; 
and the shoulder-stripe, which is sometimes double and some- 
times treble, is common; the side of the face, moreover, is 
sometimes striped. The stripes are often plainest in the foal ; 
and sometimes quite disappear in old horses. Colonel Poole 
has seen both gray and bay Kattywar horses striped when 
first foaled. I have also reason to suspect, from information 
given me by Mr. W. W. Edwards, that with the English 
race-horse the spinal stripe is much commoner in the foal 
than in the full-grown animal. I have myself recently bred 
a foal from a bay mare (offspring of a Turkoman horse and 
a Flemish mare) by a bay English race-horse; this foal when 
a week old was marked on its hinder quarters and on its 
forehead with numerous, very narrow, dark, zebra-like bars, 
and its legs were feebly striped: all the stripes soon disap- 
peared completely. Without here entering on further details, 
I may state that I have collected cases of leg and shoulder 
stripes in horses of very different breeds in various countries 
from Britain to Eastern China; and from Norway in the 
north to the Malay Archipelago in the south. In all parts of 
the world these stripes occur far oftenest in duns and mouse- 
duns; by the term dun a large range of colour is included, 
from one between brown and black to a close approach to 

I am aware that Colonel Hamilton Smith, who has written 
on this subject, believes that the several breeds of the horse 
are descended from several aboriginal species — one of which, 
the dun, was striped; and that the above-described appear- 
ances are all due to ancient crosses with the dun stock. But 
this view may be safely rejected ; for it is highly improbable 
that the heavy Belgian cart-horse, Welsh ponies, Norwegian 
cobs, the lanky Kattywar race, &c., inhabiting the most dis- 


tant parts of the world, should all have been crossed with 
one supposed aboriginal stock. 

Now let us turn to the effects of crossing the several spe- 
cies of the horse-genus. Rollin asserts, that the common 
mule from the ass and horse is particularly apt to have bars 
on its legs; according to Mr. Gosse, in certain parts of the 
United States about nine out of ten mules have striped legs. 
I once saw a mule with its legs so much striped that any one 
might have thought that it was a hybrid-zebra; and Mr. 
W. C. Martin, in his excellent treatise on the horse, has 
given a figure of a similar mule. In four coloured drawings, 
which I have seen, of hybrids between the ass and zebra, the 
legs were much more plainly barred than the rest of the 
body ; and in one of them there was a double shoulder-stripe. 
In Lord Morton's famous hybrid, from a chestnut mare and 
male quagga, the hybrid, and even the pure offspring subse- 
quently produced from the same mare by a black Arabian 
sire, were much more plainly barred across the legs than is 
even the pure quagga. Lastly, and this is another most re- 
markable case, a hybrid has been figured by Dr. Gray (and 
he informs me that he knows of a second case) from the ass 
and the hemionus; and this hybrid, though the ass only occa- 
sionally has stripes on his legs and the hemionus has none 
and has not even a shoulder-stripe, nevertheless had all four 
legs barred, and had three short shoulder-stripes, like those 
on the dun Devonshire and Welsh ponies, and even had some 
zebra-like stripes on the sides of its face. With respect to 
this last fact, I was so convinced that not even a stripe of 
colour appears from what is commonly called chance, that I 
was led solely from the occurrence of the face-stripes on 
this hybrid from the ass and hemionus to ask Colonel Poole 
whether such face-stripes ever occurred in the eminently 
striped Kattywar breed of horses, and was, as we have seen, 
answered in the affirmative. 

What now are we to say to these several facts? We see 
several distinct species of the horse-genus becoming, by 
simple variation, striped on the legs like a zebra, or striped 
on the shoulders like an ass. In the horse we see this ten- 
dency strong whenever a dun tint appears — a tint which ap- 
proaches to that of the general colouring of the other species 


of the genus. The appearance of the stripes is not accom- 
panied by any change of form or by any other new character. 
We see this tendency to become striped most strongly dis- 
played in hybrids from between several of the most distinct 
species. Now observe the case of the several breeds of 
pigeons: they are descended from a pigeon (including two or 
three sub-species or geographical races) of a bluish colour, 
with certain bars and other marks; and when any breed 
assumes by simple variation a bluish tint, these bars and 
other marks invariably reappear; but without any other 
change of form or character. When the oldest and truest 
breeds of various colours are crossed, we see a strong ten- 
dency for the blue tint and bars and marks to reappear in 
the mongrels. I have stated that the most probable hypothe- 
sis to account for the reappearance of very ancient charac- 
ters, is — that there is a tendency in the young of each suc- 
cessive generation to produce the long-lost character, and 
that this tendency, from unknown causes, sometimes prevails. 
And we have just seen that in several species of the horse- 
genus the stripes are either plainer or appear more com- 
monly in the young than in the old. Call the breeds of 
pigeons, some of which have bred true for centuries, species; 
and how exactly parallel is the case with that of the species 
of the horse-genus ! For myself, I venture confidently to 
look back thousands on thousands of generations, and I see 
an animal striped like a zebra, but perhaps otherwise very 
differently constructed, the common parents of our domestic 
horse (whether or not it be descended from one or more 
wild stocks), of the ass, the hemionus, quagga, and zebra. 

He who believes that each equine species was indepen- 
dently created, will, I presume, assert that each species has 
been created with a tendency to vary, both under nature and 
under domestication, in this particular manner, so as often 
to become striped like the other species of the genus; and 
that each has been created with a strong tendency, when 
crossed with species inhabiting distant quarters of the world, 
to produce hybrids resembling in their stripes, not their own 
parents, but other species of the genus. To admit this view 
is, as it seems to me, to reject a real for an unreal, or at 
least for an unknown, cause. It makes the works of God a 


mere mockery and deception ; I would almost as soon believe 
with the old and ignorant cosmogonists, that fossil shells had 
never lived, but had been created in stone so as to mock the 
shells living on the sea-shore. 

Summary. — Our ignorance of the laws of variation is pro- 
found. Not in one case out of a hundred can we pretend to 
assign any reason why this or that part has varied. But 
whenever we have the means of instituting a comparison, the 
same laws appear to have acted in producing the lesser differ- 
ences between varieties of the same species, and the greater 
differences between species of the same genus. Changed 
conditions generally induce mere fluctuating variability, but 
sometimes they cause direct and definite effects; and these 
may become strongly marked in the course of time, though 
we have not sufficient evidence on this head. Habit in pro- 
ducing constitutional peculiarities and use in strengthening 
and disuse in weakening and diminishing organs, appear in 
many cases to have been potent in their effects. Homologous 
parts tend to vary in the same manner, and homologous parts 
tend to cohere. Modifications in hard parts and in external 
parts sometimes affect softer and internal parts. When one 
part is largely developed, perhaps it tends to draw nourish- 
ment from the adjoining parts; and every part of the struc- 
ture v/hich can be saved without detriment will be saved. 
Changes of structure at an early age may aft'ect parts subse- 
quently developed; and many cases of correlated variation, 
the nature of which we are unable to understand, undoubt- 
edly occur. Multiple parts are variable in number and in 
structure, perhaps arising from such parts not having been 
closely specialised for any particular function, so that their 
modifications have not been closely checked by natural selec- 
tion. It follows probably from this same cause, that organic 
beings low in the scale are more variable than those stand- 
ing higher in the scale, and which have their whole organi- 
sation more specialised. Rudimentary organs, from being 
useless, are not regulated by natural selection, and hence are 
variable. Specific characters — that is, the characters which 
have come to differ since the several species of the same 
genus branched off from a common parent — are more vari- 
able than generic characters, or those which have long been 


inherited, and have not differed within this same period. In 
these remarks we have referred to special parts or organs 
being still variable, because they have recently varied and 
thus come to dift'er; but we have also seen in the second 
chapter that the same principle applies to the whole indi- 
vidual; for in a district where many species of a genus are 
found — that is, where there has been much former variation 
and differentiation, or where the manufactory of new specific 
forms has been actively at work — in that district and amongst 
these species, ws now find, on an average, most varieties. 
Secondary sexual characters are highly variable, and such 
characters differ much in the species' of the same group. 
Variability in the same parts of the organisation has gener- 
ally been taken advantage of in giving secondary sexual 
differences to the two sexes of the same species, and specific 
differences to the several species of the same genus. Any 
part or organ developed to an extraordinary size or in an 
extraordinary manner, in comparison with the same part or 
organ in the allied species, must have gone through an 
extraordinary amount of modification since the genus arose; 
and thus we can understand why it should often still be vari- 
able in a much higher degree than other parts; for variation 
is a long-continued and slow process, and natural selection 
will in such cases not as yet have had time to overcome the 
tendency to further variability and to reversion to a less 
modified state. But when a species with any extraordinarily- 
developed organ has become the parent of many modified 
descendants — which on our view must be a very slow process, 
requiring a long lapse of time — in this case, natural selection 
has succeeded in giving a fixed character to the organ, in 
however extraordinary a manner it may have been developed. 
Species inheriting nearly the same constitution from a com- 
mon parent, and exposed to similar influences, naturally tend 
to present analogous variations, or these same species may 
occasionally revert to some of the characters of their ancient 
progenitors. Although new and important modifications may 
not arise from reversion and analogous variation, such modi- 
fications will add to the beautiful and harmonious diversity 
of nature. 

Whatever the cause may be of each slight difference be- 


twcen the offspring and their parents — and a cause for each 
must exist — we have reason to beHeve that it is the steady 
accumulation of beneficial differences which has given rise 
to all the more important modifications of structure in rela- 
tion to the habits of each species. 

Difficulties of the Theory 

Difficulties of the theory of descent with modification — Absence or 
rarity of transitional varieties — Transitions in habits of life — 
Diversified habits in the same species — Species with habits widely 
different from those of their allies — Organs of extreme perfec- 
tion — Modes of transition — Cases of difficulty — Natura non facit 
saltum — Organs of small importance — Organs not in all cases 
absolutely perfect — The law of Unity of Type and of the Con- 
ditions of Existence embraced by the theory of Natural 

LONG before the reader has arrived at this part of my 
work, a crow^d of difficulties will have occurred to him. 
Some of them are so serious that to this day I can 
hardly reflect on them without being in some degree stag- 
gered; but, to the best of my judgment, the greater number 
are only apparent, and those that are real are not, I think, 
fatal to theory. 
r These difficulties and objections may be classed under the 
\ following heads; — First, why, if species have descended from 
V" other species by fine gradations, do we not everywhere see 
's innumerable transitional forms? Why is not all nature in 
}$■ confusion, instead of the species being, as we see them, well 
I defined? 

L- Secondly, is it possible that an animal having, for instance, 
the structure and habits of a bat, could have been formed by 
the modification of some other animal with widely different 
habits and structure? Can we believe that natural selection 
could produce, on the one hand, an organ of trifling impor- 
tance, such as the tail of a girafife, which serves as a fly- 
flapper, and, on the other hand, an organ so wonderful as the 

Thirdly, can instincts be acquired and modified through 
natural selection? What shall we say to the instinct which 



leads the bee to make cells, and which has practically antici- 
pated the discoveries of profound mathematicians? 

Fourthly, how can we account for species, when crossed, 
being sterile and producing sterile offspring, whereas, when 
varieties are crossed, their fertility is unimpaired? 

The two first heads will here be discussed; some miscel- 
laneous objections in the following chapter; Instinct and 
Hybridism in the two succeeding chapters. 

On the Absence or Ranty of Transitional Varieties. — As 
natural selection acts solely by the preservation of profitable 
modifications, each new form will tend in a fully-stocked 
country to take the place of, and finally to exterminate, its 
own less improved parent-form and other less-favoured forms 
with which it comes into competition. Thus extinction and 
natural selection go hand in hand. Hence, if we look at 
each species as descended from some unknown form, both 
the parent and all the transitional varieties will generally 
have been exterminated by the very process of the formation 
and perfection of the new form. 

But, as by this theory innumerable transitional forms must 
have existed, why do we not find them embedded in countless 
numbers in the crust of the earth? It will be more con- 
venient to discuss this question in the chapter on the Imper- 
fection of the Geological Record; and I will here only state / 
that I believe the answer mainly lies in the record being in- ■^ 
parably less perfect than is generally supposed. The crust 
of the earth is a vast museum; but the natural collections 
have been imperfectly made, and only at long intervals of 

But it may be urged that when several closely-allied 
species inhabit the same territory, we surely ought to find at 
the present time many transitional forms. Let us take a 
simple case : in travelling from north to south over a conti- 
nent, we generally meet at successive intervals with closely 
allied or representative species, evidently filling nearly the 
same place in the natural economy of the land. Those represen- 
tative species often meet and interlock; and as the one be- 
comes rarer and rarer, the other becomes more and more 
frequent, till the one replaces the other. But if we compare 
these species where they intermingle, they are generally as ab- 


solutely distinct from each other in every detail of structure 
as are specimens taken from the metropolis inhabited by each. 
By my theory these allied species are descended from a com- 
mon parent; and during the process of modification, each has 
become adapted to the conditions of life of its own region, 

^''and has supplanted and exterminated its original parent- 
form and all the transitional varieties between its past and 
present states. Hence we ought not to expect at the present 

f"~-i:ime to meet with numerous transitional varieties in each re- 
gion, though they must have existed there, and may be em- 
bedded there in a fossil condition. But in the intermediate 
region, having intermediate conditions of life, why do we not 
how find closely-linking intermediate varieties? This diffi- 
culty for a long time quite confounded me. But I think it 
can be in large part explained. 

In the first place we should be extremely cautious in in- 
ferring, because an area is now continuous, that it has been 
continuous during a long period. Geology would lead us to 
believe that most continents have been broken up into islands 
even during the later tertiary periods; and in such islands 
distinct species might have been separately formed without 
the possibility of intermediate varieties existing in the inter- 
mediate zones. By changes in the form of the land and of 
climate, marine areas now continuous must often have ex- 
isted within recent times in a far less continuous and uniform 
condition than at present. But I will pass over this way of 
escaping from the difficulty; for I believe that many per- 
fectly defined species have been formed on strictly continu- 
ous areas ; though I do not doubt that the formerly broken 
condition of areas now continuous, has played an important 
part in the formation of new species, more especially with 
freely-crossing and wandering animals. 

In looking at species as they are now distributed over a 
wide area, we generally find them tolerably numerous over a 
large territory, then becoming somewhat abruptly rarer and 
rarer on the confines, and finally disappearing. Hence the 
neutral territory between two representative species is gen- 
erally narrow in comparison with the territory proper to each. 
We see the same fact in ascending mountains, and sometimes 
it is quite remarkable how abruptly, as Alph. de Candolle has 


observed, a common alpine species disappears. The same 
fact has been noticed by E. Forbes in sounding the depths of 
the sea with the dredge. To those who look at climate and 
the physical conditions of life as the all-important elements 
of distribution, these facts ought to cause surprise, as cli- 
mate and height or depth graduate away insensibly. But 
when we bear in mind that almost every species, even in its 
metropolis, would increase immensely in numbers, were it not 
for other competing species; that nearly all either prey on or 
serve as prey for others ; in short, that each organic being is 
either directly or indirectly related in the most important 
manner to other organic beings, — we see that the range of the 
inhabitants of any country by no means exclusively depends 
on insensibly changing physical conditions, but in a large 
part on the presence of other specie?, c:-' which it lives, or by 
which it is destroyed, or with which it comes into competi- 
tion; and as these species are already defined objects, not 
blending one into another by insensible gradations, the range 
of any one species, depending as it does on the range of 
others, will tend to be sharply defined. Moreover, each 
species on the confines of its range, where it exists in less- 
ened numbers, will, during fluctuations in the number of its 
enemies or of its prey, or in the nature of the seasons, be ex- 
tremely liable to utter extermination; and thus its geographi- 
cal range will come to be still more sharply defined. 

As allied or representative species, when inhabiting a con- 
tinuous area, are generally distributed in such a manner that 
each has a wide range, with a comparatively narrow neutral 
territory between them, in which they become rather suddenly 
rarer and rarer; then, as varieties do not essentially differ 
from species, the same rule will probably apply to both ; and 
if we take a varying species inhabiting a very large area, we 
shall have to adapt two varieties to two large areas, and a 
third variety to a narrow intermediate zone. The intermedi- 
ate variety, consequently, will exist in lesser numbers from 
inhabiting a narrow and lesser area; and practically, as far as 
I can make out, this rule holds good with varieties in a state 
of nature. I have met with striking instances of the rule in 
the case of varieties intermediate between well-marked vari- 
eties in the genus Balanus. And it would appear from infor- 


mation given me by Mr. Watson, Dr. Asa Gray, and Mr. 
Vv'^ollaston, that generally, when varieties intermediate be- 
jtween two_other forms occurTTKey^^Tejnuch rarer numeri- 
'cally than the forms which they connect. Now, if we may 
trust these facts and inferences, and conclude that varieties 
linking two other varieties together generally have existed 
in lesser numbers than the forms which they connect, then 
we can understand why intermediate varieties should not en- 
dure for very long periods :— why, as a general rule, they 
should be exterminated and disappear, sooner than the forms 
wEicbthey^'origiirany linked togetherT^ 

For any form existing in lesser numbers would, as already 
remarked, run a greater chance of being exterminated than 
one existing in large numbers ; and in this particular case the 
intermediate form would be eminently liable to the inroads of 
closely-allied forms existing on both sides of it. But it is a 
far more important consideration, that during the process of 
further modification, by which two varieties are supposed to 
be converted and perfected into two distinct species, the two 
which exist in larger numbers, from inhabiting larger areas, 
will have a great advantage over the intermediate variety, 
which exists in smaller numbers in a narrow and intermedi- 
ate zone. For forms existing in larger numbers will have a 
better chance, within any given period, of presenting further 
favourable variations for natural selection to seize on, than 
will the rarer forms which exist in lesser numbers. Hence, 
the more common forms, in the race for life, will tend to beat 
and supplant the less common forms, for these will be more 
slowly modified and improved. It is the same principle 
which, as I believe, accounts for the common species in each 
country, as shown in the second chapter, presenting on an 
average a greater number of well-marked varieties than do 
the rarer species. I may illustrate what I mean by supposing 
three varieties of sheep to be kept, one adapted to an exten- 
sive mountainous region ; a second to a comparatively narrow, 
hilly tract; and a third to the wide plains at the base; and 
that the inhabitants are all trying with equal steadiness and 
skill to improve their stocks by selection ; the chances in this 
case will be strongly in favour of the great holders on the 
mountains or on the plains, improving their breeds more 


quickly than the small holders on the intermediate narrow, 
hilly tract; and consequently the improved mountain or plain 
breed will soon take the place of the less improved hill breed ; 
and thus the two breeds, which originally existed in greater 
numbers, will come into close contact with each other, with- 
out the interposition of the supplanted, intermediate hill 

To sum up, I believe that species come to be tolerably well- 
defined objects, and do not at any one period present an inex- 
tricable chaos of varying and intermediate links : first, be- 
cause new varieties are very slowly formed, for variation is 
a slow process, and natural selection can do nothing until 
favourable individual differences or variations occur, and un- 
til a place in the natural polity of the country can be better 
filled by some modification of some one or more of its inhabit- 
ants. And such new places will depend on slow changes of 
climate, or on the occasional immigration of new inhabitants, 
and, probably, in a still more important degree, on some of 
the old inhabitants becoming slowly modified, with the new 
forms thus produced and the old ones acting and reacting on 
each other. So that, in any one region and at any one time, 
we ought to see only a few species presenting slight modifi- 
cations of structure in some degree permanent; and this as- 
suredly we do see. 

Secondly, areas now continuous must often have existed 
within the recent period as isolated portions, in which many 
forms, more especially amongst the classes which unite for 
each birth and wander much, may have separately been ren- 
dered sufficiently distinct to rank as representative species. 
In this case, intermediate varieties between the several repre- 
sentative species and their common parent, must formerly 
have existed within each isolated portion of the land, but 
these links during the process of natural selection will have 
been supplanted and exterminated, so that they will no longer 
be found in a living state. 

Thirdly, when two or more varieties have been formed in 
different portions of a strictly continuous area, intermediate 
varieties will, it is probable, at first have been formed in the 
intermediate zones, but they will generally have had a short 
duration. For these intermediate varieties will, from reasons 


already assigned (namely from what we know of the actual 
distribution of closely allied or representative species, and 
likewise of acknowledged varieties), exist in the intermediate 
zones in lesser numbers than the varieties which they tend to 
connect. From this cause alone the intermediate varieties 
will be liable to accidental extermination; and during the 
process of further modification through natural selection, 
they will almost certainly be beaten and supplanted by the 
forms which they connect ; for these from existing in greater 
numbers will, in the aggregate, present more varieties, and 
thus be further improved through natural selection and gain 
further advantages. 

Lastly, looking not to any one time, but to all time, if my 
theory be true, numberless intermediate varieties, linking 
closely together all the species of the same group, must as- 
suredly have existed ; but the very process of natural selec- 
tion constantly tends, as has been so often remarked, to ex- 
terminate the parent-forms and the intermediate links. Con- 
sequently evidence of their former existence could be found 
only amongst fossil remains, which are preserved, as we shall 
attempt to show in a future chapter, in an extremely imper- 
fect and intermittent record. 

On the Origin and Transitions of Organic Beings zvith 
peculiar Habits and Structure. — It has been asked by the 
opponents of such views as I hold, how, for instance, could 
a land carnivorous animal have been converted into one with 
aquatic habits ; for how could the animal in its transitional 
state have subsisted? It would be easy to show that there 
now exist carnivorous animals presenting close intermediate 
grades from strictly terrestrial to aquatic habits ; and as each 
exists by a struggle for life, it is clear that each must be well 
adapted to its place in nature. Look at the Mustela vison 
of North America, which has webbed feet, and which re- 
sembles an otter in its fur, short legs, and form of tail. Dur- 
ing the summer this animal dives for and preys on fish, but 
during the long winter it leaves the frozen waters, and preys, 
like other pole-cats, on mice and land animals. If a different 
case had been taken^ and it had been asked how an insectiv- 
orous quadruped could possibly have been converted into ■ 
a flying bat, the question would have been far more 


difficult to answer. Yet I think such difficulties have little 

Here, as on other occasions, I lie under a heavy disadvan- 
tage, for, out of the many striking cases which I have col- 
lected, I can give only one or two instances of transitional 
habits and structures in allied species ; and of diversified 
habits, either constant or occasional, in the same species. 
And it seems to me that nothing less than a long list of such 
cases is sufficient to lessen the difficulty in any particular 
case like that of the bat. 

Look at the family of squirrels; here we have the finest 
gradation from animals with their tails only slightly flat- 
tened, and from others, as Sir J. Richardson has remarked, 
with the posterior part of their bodies rather wide and with 
the skin on their flanks rather full, to the so-called flying 
squirrels ; and flying squirrels have their limbs and even the 
base of the tail united by a broad expanse of skin, which 
serves as a parachute and allows them to glide through the 
air to an astonishing distance from tree to tree. We cannot 
doubt that each structure is of use to each kind of squirrel in 
its own country, by enabling it to escape birds or beasts of 
prey, to collect food more quickly, or, as there is reason to 
believe, to lessen the danger from occasional falls. But it 
does not follow from this fact that the structure of each 
squirrel is the best that it is possible to conceive under all 
possible conditions. Let the climate and vegetation change, 
let other competing rodents or new beasts of prey immigrate, 
or old ones become modified, and all analogy would lead us to 
believe that some at least of the squirrels would decrease in 
numbers or become exterminated, unless they also became 
modified and improved in structure in a corresponding man- 
ner. Therefore, I can see no difficulty, more especially under 
changing conditions of life, in the continued preservation of 
individuals with fuller and fuller flank-membranes, each modi- 
fication being useful, each being propagated, until, by the ac- 
cumulated effects of this process of natural selection, a per- 
fect so-called flying squirrel was produced. 

Now look at the Galeopithccus or so-called flying lemur, 
which formerly was ranked amongst bats, but is now believed 
to belong to the Insectivora. As extremely wide flank-mem- 


brane stretches from the comers of the jaw to the tail, and 
includes the limbs with the elongated fingers. This flank- 
membrane is furnished with an extensor muscle. Although 
no graduated links of structure, fitted for gliding through the 
air, now connect the Galeopithecus with the other Insec- 
tivora, yet there is no difficulty to supposing that such links 
formerly existed, and that each was developed in the same 
manner as with the less perfectly gliding squirrels ; each 
grade of structure having been useful to its possessor. Nor 
can I see any insuperable difficulty in further believing that 
the membrane connected fingers and fore-arm of the Galeopi- 
thecus might have been greatly lengthened by natural selec- 
tion; and this, as far as the organs of flight are concerned, 
would have converted the animal into a bat. In certain bats 
in which the wing-membrane extends from the top of the 
shoulder to the tail and includes the hind-legs, we perhaps 
see traces of an apparatus originally fitted for gliding 
through the air rather than for flight. 

If about a dozen genera of birds were to become extinct, 
who would have ventured to surmise that birds might have 
existed which used their wings solely as flappers, like the 
logger-headed duck (Micropterus of Eyton) ; as fins in the 
water and as front-legs on the land, like the penguin ; as 
sails, like the ostrich ; and functionally for no purpose, like 
the Apteryx? Yet the structure of each of these birds is 
good for it, under the conditions of life to which it is exposed, 
for each has to live by a struggle ; but it is not necessarily 
the best possible under all possible conditions. It must not 
be inferred from these remarks that any of the grades of 
wing-structure here alluded to, which perhaps may all be the 
result of disuse, indicate the steps by which birds actually 
acquired their perfect power of flight ; but they serve to show 
what diversified means of transition are at least possible. 

Seeing that a few members of such water-breathing classes 
as the Crustacea and Mollusca are adapted to live on the* 
land ; and seeing that we have flying birds and mammals, fly- 
ing insects of the most diversified types, and formerly had 
flying reptiles, it is conceivable that flying-fish, which now 
glide far through the air, slightly rising and turning by the 
aid of their fluttering fins, might have been modified ^x^to per- 


'fectly winged animals. If this had been cfTcctcd, who would 
have ever imagined that in an early transitional state they 
had been the inhabitants of the open ocean, and had used 
their incipient organs of flight exclusively, as far as we know, 
to escape being devoured by other fish? 

When we see any structure highly perfected for any par- 
ticular habit, as the wings of a bird for flight, we should bear 
in mind that animals displaying early transitional grades of 
the structure will seldom have survived to the present day, 
for they will have been supplanted by their successors, which 
were gradually rendered more perfect through natural selec- 
tion. Furthermore, we may conclude that transitional states 
between structures fitted for very different habits of life will 
rarely have been developed at an early period in great num- 
bers and under many subordinate forms. Thus, to return to our 
imaginary illustration of the flying-fish, it does not seem 
probable that fishes capable of true flight would have been 
developed under many subordinate forms, for taking prey of 
many kinds in many ways, on the land and in the water, until 
their organs of flight had come to a high state of perfection, 
so as to have given them a decided advantage over other ani- 
mals in the battle for life. Hence the chance of discovering 
species with transitional grades of structure in a fossil con- 
dition will always be less, from their having existed in lesser 
numbers, than in the case of species with fully developed 

I will now give two or three instances both of diversified 
and of changed habits in the individuals of the same species. 
In either case it would be easy for natural selection to adapt 
the structure of the animal to its changed habits, or exclu- 
sively to one of its several habits. It is, however, difficult 
to decide, and immaterial for us, whether habits generally 
change first and structure afterwards; or whether slight 
modifications of structure lead to changed habits ; both prob- 
ably often occurring almost simultaneously. Of cases of 
changed habits it will sufiice merely to allude to that of the 
many British insects which now feed on exotic plants, or ex- 
clusively on artificial substances. Of diversified habits innu- 
merable instances could be given : I have often watched a 
tyrant flycatcher (Saurophagus sulphuratus) in South Amer- 


ica, hovering over one spot and then proceeding to another, 
like a kestrel, and at other times standing stationary on the 
margin of water, and then dashing into it like a kingfisher 
at a fish. In our own country the larger titmouse (Parus 
major) may be seen climbing branches, almost like a creeper; 
it sometimes, like a shrike, kills small birds by blows on the 
head; and I have many times seen and heard it hammering 
the seeds of the yew on a branch, and thus breaking them 
like a nuthatch. In North America the black bear was seen 
by Hearne swimming for hours with widely open mouth, thus 
catching, almost like a whale, insects in the water. 

As we sometimes see individuals following habits different 
from those proper to their species and to the other species of 
the same genus, we might expect that such individuals would 
occasionally give rise to new species, having anomalous 
habits, and with their structure either slightly or considerably 
modified from that of their type. And such instances occur 
in nature. Can a more striking instance of adaptation be 
given than that of a woodpecker for climbing trees and seiz- 
ing insects in the chinks of the bark ? Yet in North America 
there are woodpeckers which feed largely on fruit, and others 
with elongated wings which chase insects on the wing. On 
the plains of La Plata, where hardly a tree grows, there is a 
woodpecker (Colaptes campestris) which has two toes before 
and two behind, a long pointed tongue, pointed tail-feathers, 
sufficiently stiff to support the bird in a vertical position on 
a post, but not so stiff as in the typical w^oodpeckers, and a 
straight strong beak. The beak, however, is not so straight 
or so strong as in the typical woodpeckers, but it is strong 
enough to bore into wood. Hence this Colaptes in all the 
essential parts of its structure is a woodpecker. Even in 
such trifling characters as the colouring, the harsh tone of 
the voice, and undulatory flight, its close blood-relationship 
to our common woodpecker is plainly declared; yet, as I can 
assert, not only from my own observations, but from those 
of the accurate Azara, in certain large districts it does not 
climb trees, and it makes its nest in holes in banks ! In cer- 
tain other districts, however, this same woodpecker, as Mr. 
Hudson states, frequents trees, and bores holes in the trunk 
for its nest. I may mention as another illustration of the 


varied habits of this genus, that a Mexican Colaptes has been 
described by De Saussure as boring holes into hard wood in 
order to lay up a store of acorns. 

Petrels are the most aerial and oceanic of birds, but in the 
quiet sounds of Tierra del Fuego, the Puffinuria berardi, in 
its general habits, in its astonishing power of diving, in its 
manner of swimming and of flying when made to take flight, 
would be mistaken by any one for an auk or a grebe; never- 
theless it is essentially a petrel, but with many parts of its 
organisation profoundly modified in relation to its new habits 
of life; whereas the woodpecker of La Plata has had its 
structure only slightly modified. In the case of the water- 
ouzel, the acutest observer by examining its dead body would 
never have suspected its sub-aquatic habits; yet this bird, 
which is allied to the thrush family, subsists by diving — using 
its wings under water, and grasping stones with its feet. All 
the members of the great order of Hymenopterous insects 
are terrestrial, excepting the genus Proctotrupes, which Sir 
John Lubbock has discovered to be aquatic in its habits ; it 
often enters the water and dives about by the use not of its 
legs but of its wings, and remains as long as four hours be- 
neath the surface; yet it exhibits no modification in structure 
in accordance with its abnormal habits. 

He who believes that each being has been created as we 
now see it, must occasionally have felt surprise when he has 
met with an animal having habits and structure not in agree- 
ment. What can be plainer than that the webbed feet of 
ducks and geese are formed for swimming? Yet there are up- 
land geese with webbed feet which rarely go near the water ; 
and no one except Audubon has seen the frigate-bird, which 
has all its four toes webbed, alight on the surface of the 
ocean. On the other hand, grebes and coots are eminently 
aquatic, although their toes are only bordered by membrane. 
What seems plainer than that the long toes, not furnished 
with membrane of the Grallatores are formed for walking 
over swamps and floating plants? — the water-hen and land- 
rail are members of this order, yet the first is nearly as 
aquatic as the coot, and the second nearly as terrestrial as 
the quail or partridge. In such cases, and many others could 
be given, habits have changed without a corresponding change 


of structure. The webbed feet of the upland goose may be 
said to have become almost rudimentary in function, though 
not in structure. In the frigate-bird, the deeply scooped 
membrane between the toes shows that structure has begun 
to change. 

He who believes in separate and innumerable acts of cre- 
ation may say, that in these cases it has pleased the Creator 
to cause a being of one type to take the place of one belonging 
to another type ; but this seems to me only re-stating the fact 
in dignified language. He who believes in the struggle for 
existence and in the principle of natural selection, will ac- 
knowledge that every organic being is constantly endeavour- 
ing to increase in numbers ; and that if any one being varies 
ever so little, either in habits or structure, and thus gains an 
advantage over some other inhabitant of the same country, it 
will seize on the place of that inhabitant, however different 
that may be from its own place. Hence it will cause him no 
surprise that there should be geese and frigate-birds with 
webbed feet, living on the dry land and rarely alighting on 
the water, that there should be long-toed corncrakes, living in 
meadows instead of in swamps; that there should be wood- 
peckers where hardly a tree grows ; that there should be div- 
ing thrushes and diving Hymenoptera, and petrels with the 
habits of auks. 


To suppose that the eye with all its inimitable contrivances 
for adjusting the focus to different distances, for admitting 
different amounts of light, and for the correction of spherical 
and chromatic aberration, could have been formed by natural 
selection, seems, I freely confess, absurd in the highest de- 
gree. When it was first said that the sun stood still and the 
world turned round, the common sense of mankind declared 
the doctrine false ; but the old saying of Vox populi, vox Dei, 
as every philosopher knows, cannot be trusted in science. 
Reason tells me, that if numerous gradations from a simple 
and imperfect eye to one complex and perfect can be shown 
to exist, each grade being useful to its possessor, as is cer- 
tainly the case; if further, the eye ever varies and the van- 


ations be inherited, as is likewise certainly the case; and if 
such variations should be useful to any animal under chang- 
ing conditions of life, then the difficulty of believing that a 
perfect and complex eye could be formed by natural selection, 
though insuperable by our imagination, should not be consid- 
ered as subversive of the theory. How a nerve comes to be 
sensitive to light, hardly concerns us more than how life it- 
self originated ; but I may remark that, as some of the lowest 
organisms, in which nerves cannot be detected, are capable of 
perceiving light, it docs not seem impossible that certain sen- 
sitive elements in their sarcode should become aggregated 
and developed into nerves, endowed with this special sensi- 

In searching for the gradations through which an organ in 
any species has been perfected, we ought to look exclusively 
to its lineal progenitors; but this is scarcely ever possible, 
and we are forced to look to other species and genera of the 
same group, that is to the collateral descendants from the 
same parent-form, in order to see what gradations are pos- 
sible, and for the chance of some gradations having been 
transmitted in an unaltered or little altered condition. But 
the state of the same organ in distinct classes may incident- 
ally throw light on the steps by which it has been perfected. 

The simplest organ which can be called an eye consists of 
an optic nerve, surrounded by pigment-cells and covered by 
translucent skin, but without any lens or other refractive 
body. We may, however, according to M. Jourdain. descend 
even a step lower and find aggregates of pigment-cells, appar- 
ently serving as organs of vision, without any nerves, and 
resting merely on sarcodic tissue. Eyes of the above simple 
nature are not capable of distinct vision, and serve only to 
distinguish light from darkness. In certain star-fishes, small 
depressions in the layer of pigment which surrounds the 
nerve are filled, as described by the author just quoted, with 
transparent gelatinous matter, projecting with a convex sur- 
face, like the cornea in the higher animals. He suggests that 
this serves not to form an image, but only to concentrate the 
luminous rays and render their perception more easy. In 
this concentration of the rays we gain the first and by far the 
most important step towards the fomiation of a true, picture- 


forming eye ; for we have only to place the naked extremity 
of the optic nerve, which in some of the lower animals lies 
deeply buried in the body, and in some near the surface, at 
the right distance from the concentrating apparatus, and an 
image will be formed on it. 

In the great class of the Articulata, we may start from an 
optic nerve simply coated with pigment, the latter sometimes 
forming a sort of pupil, but destitute of a lens or other opti- 
cal contrivance. With insects it is now known that the nu- 
merous facets on the cornea of their great compound eyes 
form true lenses, and that the cones include curiously modi- 
fied nervous filaments. But these organs in the Articulata 
are so much diversified that Miiller formerly made three main 
classes with seven subdivisions, besides a fourth main class 
of aggregated simple eyes. 

When we reflect on these facts, here given much too briefly, 
with respect to the wide, diversified, and graduated range of 
structure in the eyes of the lower animals ; and when we bear 
in mind how small the number of all living forms must be in 
comparison with those which have become extinct, the diffi- 
culty ceases to be very great in believing that natural selec- 
tion may have converted the simple apparatus of an optic 
nerve, coated with pigment and invested by transparent mem- 
brane, into an optical instrument as perfect as is possessed 
by any member of the Articulate Class. 

He who will go thus far, ought not to hesitate to go one 
step further, if he finds on finishing this volume that large 
bodies of facts, otherwise inexplicable, can be explained by 
the theory of modification through natural selection ; he ought 
to admit that a structure even as perfect as an eagle's eye 
might thus be formed, although in this case he does not know 
the transitional states. It has been objected that in order to 
modify the eye and still preserve it as a perfect instrument, 
many changes would have to be effected simultaneously, 
which, it is assumed, could not be done through natural 
selection ; but as I have attempted to show in my work on the 
variation of domestic animals, it is not necessary to suppose 
that the modifications were all simultaneous, if they were ex- 
tremely slight and gradual. Different kinds of modification 
would, also, serve for the same general purpose : as Mr. Wal- 


lace has remarked, '"if a lens has too short or too long a 
focus, it may be amended either by an alteration of curvature, 
or an alteration of density; if the curvature be irregular, and 
the rays do not converge to a point, then any increased regu- 
larity of curvature will be an improvement. So the contrac- 
tion of the iris and the muscular movements of the eye are 
neither of them essential to vision, but only improvements 
which might have been added and perfected at any stage of 
the construction of the instrument." Within the highest di- 
vision of the animal kingdom, namely, the Vertebrata. we can 
start from an eye so simple, that it consists, as in the lance- 
let, of a little sack of transparent skin, furnished with a 
nerve and lined with pigment, but destitute of any other ap- 
paratus. In fishes and reptiles, as Owen has remarked, "the 
range of gradations of dioptric structures is very great." It 
is a significant fact that even in man, according to the high 
authority of Virchow, the beautiful crystalline lens is formed 
in the embryo by an accumulation of epidermic cells, lying in 
a sack-like fold of the skin ; and the vitreous body is formed 
from embryonic sub-cutaneous tissue. To arrive, however, 
at a just conclusion regarding the formation of the eye, with 
all its marvellous yet not absolutely perfect characters, it is 
indispensable that the reason should conquer the imagination; 
but I have felt the difficulty far too keenly to be surprised at 
others hesitating to extend the principle of natural selection 
to so startling a length. 

It is scarcely possible to avoid comparing the eye with a 
telescope. We know that this instrument has been perfected 
by the long-continued efforts of the highest human intellects ; 
and we naturally infer that the eye has been formed by a 
somewhat analogous process. But may not this inference be 
presumptuous? Have we any right to assume that the Cre- 
ator works by intellectual powers like those of man? If we 
must compare the eye to an optical instrument, we ought in 
imagination to take a thick layer of transparent tissue, with 
spaces filled with fluid, and with a nerve sensitive to light be- 
neath, and then suppose every part of this layer to be con- 
tinually changing slowly in density, so as to separate into 
layers of different densities and thicknesses, placed at dillor- 
ent distances from each other, and with tlie surfaces of each 

G— lie XI 


layer slowly changing in form. Further we must suppose 
that there is a power, represented by natural selection or the 
survival of the fittest, always intently watching each slight 
alteration in the transparent layers ; and carefully preserving 
each which, under varied circumstances, in any way or in any 
degree, tends to produce a distincter image. We must sup- 
pose each new state of the instrument to be multiplied by the 
million; each to be preserved until a better one is produced, 
and then the old ones to be all destroyed. In living bodies, 
variation will cause the slight alterations, generation will mul- 
tiply them almost infinitely, and natural selection will pick 
out with unerring skill each improvement. Let this process 
go on for millions of years ; and during each year on millions 
of individuals of many kinds; and may we not believe that a 
living optical instrument might thus be formed as superior 
to one of glass, as the works of the Creator are to those of 


If it could be demonstrated that any complex organ ex- 
isted, which could not possibly have been formed by numer- 
ous, successive, slight modifications, my theory would abso- 
lutely break down. But I can find out no such case. No 
doubt many organs exist of which we do not know the tran- 
sitional grades, more especially if we look to much-isolated 
species, round which, according to the theory, there has been 
much extinction. Or again, if we take an organ common to 
all the members of a class, for in this latter case the orean 
must have been originally formed at a remote period, since 
which all the many members of the class have been developed ; 
and in order to discover the early transitional grades through 
which the organ has passed, we should have to look to very 
ancient ancestral forms, long since become extinct. 

We should be extremely cautious in concluding that an 
organ could not have been formed by transitional gradations 
of some kind. Numerous cases could be given amongst the 
lower animals of the same organ performing at the same time 
wholly distinct functions ; thus in the larva of the dragon-fly 
and in the fish Cobites the alimentary canal respires, digests, 
and excretes. In the Hydra, the animal may be turned in- 


side out, and the exterior surface will then digest and the 
stomach respire. In such cases natural selection might spe- 
cialise, if any advantage were thus gained, the whole or part 
of an organ, which had previously performed two functions, 
for one function alone, and thus by insensible steps greatly 
change its nature. Many plants are known which regularly 
produce at the same time differently constructed flowers ; and 
if such plants were to produce one kind alone, a great change 
would be effected with comparative suddenness in the char- 
acter of the species. It is, however, probable that the two 
sorts of flowers borne by the same plant were originally dif- 
ferentiated by finely graduated steps, which may still be 
followed in some few cases. 

Again, two distinct organs, or the same organ under two 
very different forms, may simultaneously perform in the same 
individual the same function, and this is an extremely im- 
portant means of transition: to give one instance, — there are 
fish with gills or branchiae that breathe the air dissolved in 
the water, at the same time that they breathe free air in their 
swimbladders, this latter organ being divided by highly vas- 
cular partitions and having a ductus pneumaticus for the 
supply of air. To give another instance from the vegetable 
kingdom; plants climb by three distinct means, by spirally 
twining, by clasping a support with their sensitive tendrils, 
and by the emission of aerial rootlets ; these three means are 
usually found in distinct groups, but some few species exhibit 
two of the means, or even all three, combined in the same in- 
dividual. In all such cases one of the two organs might 
readily be modified and perfected so as to perform all the 
work, being aided during the progress of modification by the 
other organ; and then this other organ might be modified 
for some other and quite distinct purpose, or be wholly 

The illustration of the swimbladdcr in fishes is a good one, 
because it shows us clearly the highly important fact that an 
organ originally constructed for one purpose, namely, flota- 
tion, may be converted into one for a widely different pur- 
pose, namely, respiration. The swimbladdcr has, also, been 
worked in as an accessory to the auditory organs of certain 
fishes. All physiologists admit that the swimbladdcr is homol- 


ogous, or "ideally similar" in position and structure with the 
lungs of the higher vertebrate animals : hence there is no 
reason to doubt that the swimbladder has actually been con- 
verted into lungs, or an organ used exclusively for respi- 

According to this view it may be inferred that all verte- 
brate animals with true lungs are descended by ordinary gen- 
eration from an ancient and unknown prototype, which was 
furnished with a floating apparatus or swimbladder. We can 
thus, as I infer from Owen's interesting description of these 
parts, understand the strange fact that every particle of food 
and drink which we swallow has to pass over the orifice of 
the trachea, with some risk of falling into the lungs, notwith- 
standing the beautiful contrivance by which the glottis is 
closed. In the higher Vertebrata the branchise have wholly 
disappeared — but in the embryo the slits on the sides of the 
neck and the loop-like course of the arteries still mark their 
former position. But it is conceivable that the now utterly 
lost branchise might have been gradually worked in by nat- 
ural selection for some distinct purpose : for instance, Lan- 
dois has shown that the wings of insects are developed from 
the tracheae; it is therefore highly probable that in this great 
class organs which once served for respiration have been 
actually converted into organs for flight. 

In considering transitions of organs, it is so important to 
bear in mind the probability of conversion from one function 
to another, that I will give another instance. Pedunculated 
cirripedes have two minute folds of skin, called by me the 
ovigerous frena, which serve, through the means of a sticky 
secretion, to retain the eggs until they are hatched within the 
sack. These cirripedes have no branchiae, the whole surface 
of the body and of the sack, together with the small frena, 
serving for respiration. The Balanidae or sessile cirripedes, 
on the other hand, have no ovigerous frena, the eggs lying 
loose at the bottom of the sack, within the well-enclosed shell ; 
but they have, in the same relative position with the frena, 
large, much-folded membranes, which freely communicate 
with the circulatory lacunae of the sack and body, and which 
have been considered by all naturalists to act as branchiae. 
Now I think no one will dispute that the ovigerous frena in 


the one family are strictly homologous with the branchiae of 
the other family ; indeed, they graduate into each other. 
Therefore it need not be doubted that the two little folds of 
skin, which originally served as ovigerous frena, but which, 
likewise, very slightly aided in the act of respiration, have 
been gradually converted by natural selection into branchiae, 
simply through an increase in their size and the obliteration 
of their adhesive glands. If all pedunculated cirripedes had 
become extinct, and they have suffered far more extinction 
than have sessile cirripedes, who would ever have imagined 
that the branchiae in this latter family had originally existed 
as organs for preventing the ova from being washed out of 
the sack ? 

There is another possible mode of transition, namely, 
through the acceleration or retardation of the period of re- 
production. This has lately been insisted on by Prof. Cope 
and others in the United States. It is now known that some 
animals are capable of reproduction at a very early age, be- 
fore they have acquired their perfect characters; and if this 
power became thoroughly well developed in a species, it seems 
probable that the adult stage of development would sooner or 
later be lost; and in this case, especially if the larva differed 
much from the mature form, the character of the species 
would be greatly changed and degraded. Again, not a few 
animals, after arriving at maturity, go on changing in char- 
acter during nearly their whole lives. With mammals, for 
instance, the form of the skull is often much altered with age, 
of which Dr. Murie has given some striking instances with 
seals ; every one knows how the horns of stags become more 
and more branched, and the plumes of some birds become 
more finely developed, as they grow older. Prof. Cope states 
that the teeth of certain lizards change much in shape with 
advancing years ; with crustaceans not only many trivial, but 
some important parts assume a new character, as recorded 
by Fritz Miiller, after maturity. In all such cases,— and 
many could be given,— if the age for reproduction were re- 
tarded, the character of the species, at least in its adult state, 
would be modified ; nor is it improbable that the previous and 
earlier stages of development would in some cases be burned 
through and finally lost. Whether species have often or ever 


been modified through this comparatively sudden mode of 
transition, I can form no opinion ; but if this has occurred, it 
is probable that the differences between the young and the 
mature, and between the mature and the old, were primor- 
dially acquired by graduated steps. 


Although we must be extremely cautious in concluding that 
any organ could not have been produced by successive, small 
transitional gradations, yet undoubtedly serious cases of dif- 
ficulty occur. 

One of the most serious is that of neuter insects, which are 
often differently constructed from either the males or fertile 
females ; but this case will be treated of in the next chapter. 
The electric organs of fishes offer another case of special 
difficulty; for it is impossible to conceive by what steps these 
wondrous organs have been produced. But this is not sur- 
prising, for we do not even know of what use they are. In 
the Gymnotus and Torpedo they no doubt serve as powerful 
means of defence, and perhaps for securing prey; yet in the 
Ray, as observed by Matteucci, an analogous organ in the 
tail manifests but little electricity, even when the animal is 
greatly irritated; so little, that it can hardly be of any use 
for the above purposes. Moreover, in the Ray, besides the 
organ just referred to, there is, as Dr. R. M'Donnell has 
shown, another organ near the head, not known to be elec- 
trical, but which appears to be the real homologue of the 
electric battery in the Torpedo. It is generally admitted that 
there exists between these organs and ordinary muscle a close 
analogy, in intimate structure, in the distribution of the 
nerves, and in the manner in which they are acted on by 
various reagents. It should, also, be especially observed that 
muscular contraction is accompanied by an electrical dis- 
charge; and, as Dr. Radcliffe insists, "in the electrical ap- 
paratus of the torpedo during rest, there would seem to be a 
charge in every respect like that which is met with in muscle 
and nerve during rest, and the discharge of the torpedo, in- 
stead of being peculiar, may be only another form of the dis- 
charge which attends upon the action of muscle and 


motor nerve." Beyond this we cannot at present go in the 
way of explanation; but as we know so little about the uses 
of these organs, and as wc know nothing about the habits 
and structure of the progenitors of the existing electric fishes, 
it would be extremely bold to maintain that no serviceable 
transitions are possible by which these organs might have 
been gradually developed. 

These organs appear at first to offer another and far more 
serious difficulty; for they occur in about a dozen kinds of 
fish, of which several are widely remote in their affinities. 
When the same organ is found in several members of the 
same class, especially if in members having very different 
habits of life, we may generally attribute its presence to in- 
heritance from a common ancestor; and in its absence in 
some of the members to loss through disuse or natural selec- 
tion. So that, if the electric organs had been inherited from 
some one ancient progenitor, we might have expected that 
all electric fishes would have been specially related to each 
other; but this is far from the case. Nor does geolog}' at all 
lead to the belief that most fishes formerly possessed electric 
organs, which their modified descendants have now lost. But 
when we look at the subject more closely, we find in the sev- 
eral fishes provided with electric organs, that those are situ- 
ated in different parts of the body, — that they differ in con- 
struction, as in the arrangement of the plates, and, according 
to Pacini, in the process or means by which the electricity is 
excited — and lastly, in being supplied with nerves proceeding 
from different sources, and this is perhaps the most important 
of all the differences. Hence in the several fishes furnished 
with electric organs, these cannot be considered as homol- 
ogous, but only as analogous in function. Consequently there 
is no reason to suppose that they have been inherited from a 
common progenitor; for had this been the case they would 
have closely resembled each other in all respects. Thus the 
difficulty of an organ, apparently the same, arising in several 
remotely allied species, disappears, leaving only the lesser yet 
still great difficulty; namely, by what graduated steps these 
organs have been developed in each separate group of fishes. 

The luminous organs which occur in a few insects, belong- 
ing to widely different families, and which are situated in 


different parts of the body, offer, under our present state of 
ignorance, a difficulty almost exactly parallel with that of the 
electric organs. Other similar cases could be given; for in- 
stance in plants, the very curious contrivance of a mass of 
pollen-grains, borne on a foot-stalk with an adhesive gland, 
is apparently the same in Orchis and Asclepias, — genera al- 
most as remote as is possible amongst flowering plants; but 
here again the parts are not homologous. In all cases of be- 
ings, far removed from each other in the scale of organisa- 
tion, which are furnished with similar and peculiar organs, 
it will be found that although the general appearance and 
function of the organs may be the same, yet fundamental dif- 
ferences between them can always be detected. For instance, 
the eyes of cephalopods or cuttle-fish and of vertebrate ani- 
mals appear wonderfully alike ; and in such widely sundered 
groups no part of this resemblance can be due to inheritance 
from a common progenitor. Mr. Mivart has advanced this 
case as one of special difficulty, but I am unable to see the 
force of his argument. An organ for vision must be formed 
of transparent tissue, and must include some sort of lens for 
throwing an image at the back of a darkened chamber. Be- 
yond this superficial resemblance, there is hardly any real 
similarity between the eyes of cuttle-fish and vertebrates, as 
may be seen by consulting Hensen's admirable memoir on 
these organs in the Cephalopoda. It is impossible for me 
here to enter on details, but I may specify a few of the points 
of difference. The crystalline lens in the higher cuttle-fish 
consists of two parts, placed one behind the other like two 
lenses, both having a very dift'erent structure and disposition 
to what occurs in the vertebrata. The retina is wholly dif- 
ferent, with an actual inversion of the elemental parts, and 
with a large nervous ganglion included within the mem- 
branes of the eye. The relations of the muscles are as dif- 
ferent as it is possible to conceive, and so in other points. 
Hence it is not a little difficult to decide how far even the 
same terms ought to be employed in describing the eyes of 
the Cephalopoda and Vertebrata. It is, of course, open to 
any one to deny that the eye in either case could have been 
developed through the natural selection of successive slight 
variations ; but if this be admitted in the one case, it is clearly 


possible in the other; and fundamental differences of struc- 
ture in the visual organs of two groups might have been an- 
ticipated, in accordance with this view of their manner of 
formation. As two men have sometimes independently hit 
on the same invention, so in the several foregoing cases it 
appears that natural selection, working for the good of each 
being, and taking advantage of all favourable variations, has 
produced similar organs, as far as function is concerned, in 
distinct organic beings, which owe none of their structure in 
common to inheritance from a common progenitor. 

Fritz jMiiller, in order to test the conclusions arrived at in 
this volume, has followed out with much care a nearly similar 
line of argument. Several families of crustaceans include a 
few species, possessing an air-breathing apparatus and fitted 
to live out of the water. In two of these families, which were 
more especially examined by Miiller, and which are nearly 
related to each other, the species agree most closely in all 
important characters ; namely in their sense organs, circulat- 
ing system, in the position of the tufts of hair within their 
complex stomachs, and lastly in the whole structure of the 
water-breathing branchiae, even to the microscopical hooks by 
which they are cleansed. Hence it might have been expected 
that in the few species belonging to both families which live 
on the land, the equally-important air-breathing apparatus 
would have been the same ; for why should this one apparatus, 
given for the same purpose, have been made to differ, whilst 
all the other important organs were closely similar or rather 

Fritz Miiller argues that this close similarity in so many 
points of structure must, in accordance with the views ad- 
vanced by me, be accounted for by inheritance from a com- 
mon progenitor. But as that vast majority of the species in 
the above two families, as well as most other crustaceans, 
are aquatic in their habits, it is improbable in the highest 
degree, that their common progenitor should have been 
adapted for breathing air. Miiller was thus led carefully to 
examine the apparatus in the air-breathing species; and he 
found it to differ in each in several important points, as in 
the position of the orifices, in the manner in which they are 
opened and closed, and in some accessory details. Now such 


differences are intelligible, and might even have been ex- 
pected, on the supposition that species belonging to distinct 
families had slowly become adapted to live more and more 
out of water, and to breathe the air. For these species, from 
belonging to distinct families, would have differed to a cer- 
tain extent, and in accordance with the principle that the 
nature of each variation depends on two factors, viz., the 
nature of the organism and that of the surrounding condi- 
tions, their variability assuredly would not have been exactly 
the same. Consequently natural selection would have had 
different materials or variations to work on, in order to ar- 
rive at the same functional result; and the structures thus 
acquired would almost necessarily have differed. On the 
• hypothesis of separate acts of creation the whole case re- 
mains unintelligible. This line of argument seems to have 
had great weight in leading Fritz Miiller to accept the views 
maintained by me in this volume. 

Another distinguished zoologist, the late Professor Clapa- 
rede, has argued in the same manner, and has arrived at 
the same result. He shows that there are parasitic mites 
(Acaridje), belonging to distinct sub-families and families, 
which are furnished with hair-claspers. These organs must 
have been independently developed, as they could not have 
been inherited from a common progenitor ; and in the several 
groups they are formed by the modification of the fore-legs, 
— of the hind-legs, — of the maxillae or lips, — and of append- 
ages on the under side of the hind part of the body. 

In the foregoing cases, we see the same end gained and the 
same function performed, in beings not at all or only re- 
motely allied, by organs in appearance, though not in de- 
velopment, closely similar. On the other hand, it is a com- 
mon rule throughout nature that the same end should be 
gained, even sometimes in the case of closely-related beings, 
by the most diversified means. How differently constructed 
is the feathered wing of a bird and the membrane-covered 
wing of a bat; and still more so the four wings of a butter- 
fly, the two wings of a fly, and the two wings with the elytra 
of a beetle. Bivalve shells are made to open and shut, but 
on what a number of patterns is the hinge constructed, — • 


from the long row of neatly interlocking teeth in a Nucula 
to the simple ligament of a Mussel ! Seeds are disseminated 
by their minuteness, — by their capsule being converted into a 
light balloon-like envelope, — by being embedded in pulp or 
flesh, formed of the most diverse parts, and rendered nutri- 
tious, as well as conspicuously coloured, so as to attract and 
be devoured by birds, — by having hooks and grapnels of 
many kinds and serrated awns, so as to adhere to the fur of 
quadrupeds, — and by being furnished with wings and plumes, 
as different in shape as they are elegant in structure, so as to 
be wafted by every breeze. I will give one other instance ; 
for this subject of the same end being gained by the most 
diversified means well deserves attention. Some authors 
maintain that organic beings have been formed in many ways 
for the sake of mere variety, almost like toys in a shop, but 
such a view of nature is incredible. With plants having 
separated sexes, and with those in which, though hermaphro- 
dites, the pollen does not spontaneously fall on the stigma, 
some aid is necessary for their fertilisation. With several 
kinds this is effected by the pollen-grains, which are light 
and incoherent, being blown by the wind through mere chance 
on to the stigma ; and this is the simplest plan which can 
well be conceived. An almost equally simple, though very 
different, plan occurs in many plants in which a symmetrical 
flower secretes a few drops of nectar, and is consequently 
visited by insects ; and these carry the pollen from the anthers 
to the stigma. 

From this simple stage we may pass through an inex- 
haustible number of contrivances, all for the same purpose 
and effected in essentially the same manner, but entailing 
changes in every part of the flower. The nectar may be 
stored in variously shaped receptacles, with the stamens and 
pistils modified in many ways, sometimes forming trap-like 
contrivances, and sometimes capable of neatly adapted move- 
ments through irritability or elasticity. From such structures 
we may advance till we come to such a case of extraordinary 
adaptions as that lately described by Dr. Criigcr in the 
Coryanthes. This orchid has part of its labcllum or lower 
lip hollowed out into a great bucket, into which drops of 
almost pure water continually fall from two secreting horns 


which stand above it; and when the bucket is half full, the 
water overflows by a spout on one side. The basal part of 
the labellum stands over the bucket, and is itself hollowed 
out into a sort of chamber with two lateral entrances ; with- 
in this chamber there are curious fleshy ridges. The most 
ingenious man, if he had not witnessed what takes place, 
could never have imagined what purpose all these parts serve. 
But Dr. Criiger saw crowds of large humble-bees visiting the 
gigantic flowers of this orchid, not in order to suck nectar, 
but to gnaw off the ridges within the chamber above the 
bucket ; in doing this they frequently pushed each other into 
the bucket, and their wings being thus wetted they could not 
fly away, but were compelled to crawl out through the pas- 
sage formed by the spout or overflow. Dr. Criiger saw a 
"continual procession" of bees thus crawling out of their 
involuntary bath. The passage is narrow, and is roofed over 
by the column, so that a bee, in forcing its way out, first rubs 
its back against the viscid stigma and then against the viscid 
glands of the pollen-masses. The pollen-masses are thus 
glued to the back of the bee which first happens to crawl out 
through the passage of a lately expanded flower, and are 
thus carried away. Dr. Criiger sent me a flower in spirits of 
wine, with a bee which he had killed before it had quite 
crawled out with a pollen-mass still fastened to its back. 
When the bee, thus provided, flies to another flower, or to 
the same flower a second time, and is pushed by its comrades 
into the bucket and then crawls out by the passage, the 
pollen-mass necessarily comes first into contact with the 
viscid stigma, and adheres to it, and the flower is fertilised. 
Now at last we see the full use of every part of the flower, 
of the water-secreting horns, of the bucket half full of water, 
"ivhich prevents the bees from flying away, and forces them 
to crawl out through the spout, and rub against the properly 
placed viscid pollen-masses and the viscid stigma. 

The construction of the flower in another closely allied 
orchid, namely the Catasetum, is widely different, though 
serving the same end; and is equally curious. Bees visit 
these flowers, like those of the Coryanthes, in order to gnaw 
the labellum; in doing this they inevitably touch a long, 
tapering, sensitive projection, or, as I have called it, the 


antenna. This antenna, when touched, transmits a sensation 
or vibration to a certain membrane which is instantly rup- 
tured; this sets free a spring by which the pollen-mass is shot 
forth, like an arrow, in the right direction, and adheres by 
its viscid extremity to the back of the bee. The pollen-mass 
of the male plant (for the sexes are separate in this orchid) 
is thus carried to the flower of the female plant, where it is 
brought into contact with the stigma, which is viscid enough 
to break certain elastic threads, and retaining the pollen, 
iertilisation is effected. 

How, it may be asked, in the foregoing and in innumer- 
able other instances, can we understand the graduated scale of 
complexity and the multifarious means for gaining the same 
end. The answer no doubt is, as already remarked, that when 
two forms vary, which already differ from each other in some 
slight degree, the variability will not be of the same exact 
nature, and consequently the results obtained through natural 
selection for the same general purpose will not be the same. 
We should also bear in mind that every highly developed 
organism has passed through many changes; and that each 
modified structure tends to be inherited, so that each modifi- 
cation will not readily be quite lost, but may be again and 
again further altered. Hence the structure of each part of 
each species, for whatever purpose it may serve, is the sum 
of many inherited changes, through which the species has 
passed during its successive adaptations to changed habits 
and conditions of life. 

Finally then, although in many cases it is most difficult 
even to conjecture by what transitions organs have arrived 
at their present state; yet, considering how small the propor- 
tion of living and known forms is to the extinct and un- 
known, I have been astonished how rarely an organ can be 
named, towards which no transitional grade is known to lead. 
It certainly is true, that new organs appearing as if created 
for some special purpose, rarely or never appear in any 
being; — as indeed is shown by that old, but somewhat exag- 
gerated, canon in natural history of "Natura non facit sal- 
tum." We meet with this admission in the writings of almost 
every experienced naturalist; or as Milne Edwards has well 
expressed it, Nature is prodigal in variety, but niggartl in 


innovation. Why, on the theory of Creation, should there 
be so much variety and so little real novelty? Why should 
all the parts and organs of many independent beings, each 
supposed to have been separately created for its proper place 
in nature, be so commonly linked together by graduated 
steps? Why should not Nature take a sudden leap from 
structure to structure? On the theory of natural selection, 
we can clearly understand why she should not; for natural 
selection acts only by taking advantage of slight successive 
variations; she can never take a great and sudden leap, but 
must advance by short and sure, though slow steps. 


As natural selection acts by life and death, — ^by the sur- 
vival of the fittest, and by the destruction of the less well- 
fitted individuals, — I have sometimes felt great difficulty in 
understanding the origin or formation of parts of little im- 
portance; almost as great, though of a very different kind, 
as in the case of the most perfect and complex organs. 

In the first place, we are much too ignorant in regard to 
the whole economy of any one organic being, to say what 
slight modifications would be of importance or not. In a 
former chapter I have given instances of very trifling char- 
acters, such as the down on fruit and the colour of its flesh, 
the colour of the skin and hair of quadrupeds, which, from 
being correlated with constitutional differences or from de- 
termining the attacks of insects, might assuredly be acted on 
by natural selection. The tail of the giraffe looks like an 
artificially constructed fly-flapper; and it seems at first in- 
credible that this could have been adapted for its present 
purpose by successive slight modifications, each better and 
better fitted, for so trifling an object as to drive away flies; 
yet we should pause before being too positive even in this 
case, for we know that the distribution and existence of 
cattle and other animals in South America absolutely depend 
on their power of resisting the attacks of insects : so that 
individuals which could by any means defend themselves 
from these small enemies, would be able to range into new 


pastures and thus gain a great advantage. It is not that the 
larger quadrupeds are actually destroyed (except in some 
rare cases) by flies, but they are incessantly harassed and 
their strength reduced, so that they are more subject to 
disease, or not so well enabled in a coming dearth to search 
for food, or to escape from beasts of prey. 

Organs now of trifling importance have probably in some 
cases been of high importance to an early progenitor, and, 
after having been slowly perfected at a former period, have 
been transmitted to existing species in nearly the same date, 
although now of very slight use; but any actually injurious 
deviations in their structure would of course have been 
checked by natural selection. Seeing how important an 
organ of locomotion the tail is in most aquatic animals, its 
general presence and use for many purposes in so many land 
animals, which in their lungs or modified swimbladders be- 
tray their aquatic origin, may perhaps be thus accounted 
for. A well-developed tail having been formed in an aquatic 
animal, it might subsequently come to be worked in for all 
sorts of purposes, — as a fl.y-flapper, an organ of prehension, 
or as an aid in turning, as in the case of the dog, though the 
aid in this latter respect must be slight, for the hare, with 
hardly any tail, can double still more quickly. 

In the second place, we may easily err in attributing im- 
portance to characters, and in believing that they have been 
developed through natural selection. We must by no means 
overlook the effects of the definite action of changed condi- 
tions of life, — of so-called spontaneous variations, which 
seem to depend in a quite subordinate degree on the nature 
of the conditions, — of the tendency to reversion to long-lost 
characters,— of the complex laws of growth, such as of cor- 
relation, compensation, of the pressure of one part on an- 
other, etc.,— and finally of sexual selection, by which charac- 
ters of use to one sex are often gained and then transmitted 
more or less perfectly to the other sex, though of no use 
to this sex. But structures thus indirectly gained, although 
at first of no advantage to a species, may subsequently have 
been taken advantage of by its modified descendants, under 
new conditions of life and newly acquired habits. 

If green woodpeckers alone had existed, and we did not 


know that there were many black and pied kinds, I dare say 
that we should have thought that the green colour was a 
beautiful adaptation to conceal this tree-frequenting bird 
from its enemies; and consequently that it was a character 
of importance, and had been acquired through natural selec- 
tion; as it is, the colour is probably in chief part due to 
sexual selection. A trailing palm in the Malay Archipelago 
climbs the loftiest trees by the aid of exquisitely constructed 
hooks clustered around the ends of the branches, and this 
contrivance, no doubt, is of the highest service to the plant; 
but as we see nearly similar hooks on many trees which are 
not climbers, and which, as there is reason to believe from 
the distribution of the thorn-bearing species in Africa and 
South America, serve as a defence against browsing quadru- 
peds, so the spikes on the palm may at first have been de- 
veloped for this object, and subsequently have been improved 
and taken advantage of by the plant, as it underwent further 
modification and became a climber. The naked skin on the 
head of a vulture is generally considered as a direct adapta- 
tion for wallowing in putridity; and so it may be, or it may 
possibly be due to the direct action of putrid matter; but we 
should be very cautious in drawing any such inference, when 
we see that the skin on the head of the clean-feeding male 
Turkey is likewise naked. The sutures in the skulls of young 
mammals have been advanced as a beautiful adaptation for 
aiding parturition, and no doubt they facilitate, or may be 
indispensable for this act; but as sutures occur in the skulls 
of young birds and reptiles, which have only to escape from 
a broken egg, we may infer that this structure has arisen 
from the laws of growth, and has been taken advantage of in 
the parturition of the higher animals. 

We are profoundly ignorant of the cause of each slight 
variation or individual difference; and we are immediately 
made conscious of this by reflecting on the differences be- 
tween the breeds of our domesticated animals in different 
countries, — more especially in the less civilised countries 
where there has been but little methodical selection. Animals 
kept by savages in different countries often have to struggle 
for their own subsistence, and are exposed to a certain extent 
to natural selection, and individuals with slightly different 


constitutions would succeed best under different climates. 
With cattle susceptibility to the attacks of flies is correlated 
with colour, as is the liability to be poisoned by certain 
plants; so that even colour would be thus subjected to the 
action of natural selection. Some observers are convinced 
that a damp climate affects the growth of the hair, and that 
with the hair the horns are correlated. Mountain breeds al- 
ways differ from lowland breeds ; and a mountainous country 
would probably affect the hind limbs from exercising them 
more, and possibly even the form of the pelvis; and then 
by the law of homologous variation, the front limbs and the 
head would probably be affected. The shape, also, of the 
pelvis might affect by pressure the shape of certain parts of 
the young in the womb. The laborious breathing necessary 
in high regions tends, as we have good reason to believe, 
to increase the size of the chest; and again correlation 
would come into play. The effects of lessened exercise to- 
gether with abundant food on the whole organisation is 
probably still more important; and this, as H. von Nathusius 
has lately shown in his excellent Treatise, is apparently one 
chief cause of the great modification which the breed of 
swine have undergone. But we are far too ignorant to specu- 
late on the relative importance of the several known and un- 
known causes of variation ; and I have made these remarks 
only to show that, if we are unable to account for the char- 
acteristic differences of our several domestic breeds, which 
nevertheless are generally admitted to have arisen through 
ordinary generation from one or a few parent-stocks, we 
ought not to lay too much stress on our ignorance of the pre- 
cise cause of the slight analogous differences between true 



The foregoing remarks lead me to say a few words on the 
protest lately made by some naturalists, against the utilitarian 
doctrine that every detail of structure has been produced for 
the good of its possessor. They believe that many structures 
have been created for the sake of beauty, to delight man or 


the Creator (but this latter point is beyond the scope of 
scientific discussion), or for the sake of mere variety, a view 
already discussed. Such doctrines, if true, would be abso- 
lutely fatal to my theory. I fully admit that many structures 
are now of no direct use to their possessors, and may never 
have been of any use to their progenitors; but this does not 
prove that they were formed solely for beauty or variety. 
No doubt the definite action of changed conditions, and the 
various causes of modifications, lately specified, have all 
produced an effect, probably a great effect, independently of 
any advantage thus gained. But a still more important con- 
sideration is that the chief part of the organisation of every 
living creature is due to inheritance ; and consequently, 
though each being assuredly is well fitted for its place in 
nature, many structures have now no very close and direct 
relation to present habits of life. Thus, we can hardly be- 
lieve that the webbed feet of the upland goose or of the 
frigate-bird are of special use to these birds; we cannot be- 
lieve that the similar bones in the arm of the monkey, in the 
fore-leg of the horse, in the wing of the bat, and in the 
flipper of the seal, are of special use to these animals. We 
may safely attribute these structures to inheritance. But 
webbed feet no doubt were as useful to the progenitor of 
the upland goose and of the frigate-bird, as they now are to 
the most aquatic of living birds. So we may believe that the 
progenitor of the seal did not possess a flipper, but a foot 
with five toes fitted for walking or grasping; and we may 
further venture to believe that the several bones in the limbs 
of the monkey, horse, and bat, were originally developed, 
on the principle of utility, probably through the reduction of 
more .numerous bones in the fin of some ancient fish-like 
progenitor of the whole class. It is scarcely possible to de^ 
cide how much allowance ought to be made for such causes 
of change, as the definite action of external conditions, so- 
called spontaneous variations, and the complex laws of 
growth ; but with these important exceptions, we may con- 
clude that the structure of every living creature either now 
is, or was formerly, of some direct or indirect use to its 

With respect to the belief that organic beings have been 


created beautiful for the delight of man, — a belief wliich it 
has been pronounced is subversive of my whole theory, — I 
may first remark that the sense of beauty obviously depends 
on the nature of the mind, irrespective of any real quality 
in the admired object; and that the idea of what is beautiful, 
is not innate or unalterable. We see this, for instance, in 
the men of different races admiring an entirely different 
standard of beauty in their women. If beautiful objects had 
been created solely for man's gratification, it ought to be 
shown that before man appeared, there was less beauty on 
the face of the earth than since he came on the stage. Were 
the beautiful volute and cone shells of the Eocene epoch, and 
the gracefully sculptured ammonites of the Secondary period, 
created that man might ages afterwards admire them in his 
cabinet? Few objects are more beautiful than the minute 
siliceous cases of the diatomaceae: were these created that 
they might be examined and admired under the higher 
powers of the microscope? The beauty in this latter case, 
and in many others, is apparently wholly due to symmetry of 
growth. Flowers rank amongst the most beautiful produc- 
tions of nature; but they have been rendered conspfcuous in 
contact with the green leaves, and in consequence at the 
same time beautiful, so that they may be easily observed by 
insects. I have come to this conclusion from finding it an 
invariable rule that when a flower is fertilised by the wind 
it never has a gaily-coloured corolla. Several plants habitu- 
ally produce two kinds of flowers; one kind open and col- 
oured so as to attract insects; the other closed, not coloured, 
destitute of nectar, and never visited by insects. Hence we 
may conclude that, if insects had not been developed on the 
face of the earth, our plants would not have been decked with 
beautiful flowers, but would have produced only such poor 
flowers as we see on our fir, oak, nut and ash trees, on 
grasses, spinach, docks, and nettles, which are all fertilised 
through the agencv of the wind. A similar line of argument 
holds°good with 'fruits; that a ripe strawberry or cherry 
is as pleasing to the eye as to the palate— that tiie gaily- 
coloured fruit of the spindle-wood tree and the scarlet ber- 
ries of the holly are beautiful objects.— will be admitted by 
every one. But this beauty serves merely as a guide to birds 


and beasts, in order that the fruit may be devoured and the 
manured seeds disseminated : I infer that this is the case 
from having as yet found no exception to the rule that seeds 
are always thus disseminated when embedded within a fruit 
of any kind (that is within a fleshy or pulpy envelope), if it 
be coloured of any brilliant tint, or rendered conspicuous by 
being white or black. 

On the other hand, I willingly admit that a great number 
of male animals, as all our most gorgeous birds, some fishes, 
reptiles, and mammals, and a host of magnificently coloured 
butterflies, have been rendered beautiful for beauty's sake; 
but this has been effected through sexual selection, that is, by 
the more beautiful males having been continually preferred 
by the females, and not for the delight of man. So it is with 
the music of birds. We may infer from all this that a nearly 
similar taste for beautiful colours and for musical sounds 
runs through a large part of the animal kingdom. When 
the female is as beautifully coloured as the male, which is 
not rarely the case with birds and butterflies, the cause ap- 
parently lies in the colours acquired through sexual selection 
having been transmitted to both sexes, instead of to the 
males alone. How the sense of beauty in its simplest form^ 
that is, the reception of a peculiar kind of pleasure from 
certain colours, forms, and sounds — was first developed in 
the mind of man and of the lower animals, is a very obscure 
subject. The same sort of difficulty is presented, if we en- 
quire how it is that certain flavours and odours give pleasure, 
and others displeasure. Habit in all these cases appears to 
have come to a certain extent into play; but there must be 
some fundamental cause in the constitution of the nervous 
system in each species. 

Natural selection cannot possibly produce any modifica- 
tion in a species exclusively for the good of another species ; 
though throughout nature one species incessantly takes ad- 
vantage of, and profits by, the structures of others. But 
natural selection can and does often produce structures for 
the direct injury of other animals, as we see in the fang of 
the adder, and in the ovipositor of the ichneumon, by which 
its eggs are deposited in the living bodies of other insects. 
If it could be proved that any part of the structure of any 


one species had been formed for the exclusive good of an- 
other species, it would annihilate my theory, for such could 
not have been produced through natural selection. Although 
many statements may be found in works on natural history 
to this effect, I cannot find even one which seems to mc of 
any weight. It is admitted that the rattlesnake has a poison- 
fang for its own defence, and for the destruction of its prey; 
but some authors suppose that at the same time it is furnished 
with a rattle for its own injury, namely, to warn its prey. 
I would almost as soon believe that the cat curls the end 
of its tail when preparing to spring, in order to warn the 
doomed mouse. It is a much more probable view that the 
rattlesnake uses its rattle, the cobra expands its frill, and 
the puff-adder swells whilst hissing so loudly and harshly, 
in order to alarm the many birds and beasts which are known 
to attack even the most venomous species. Snakes act on the 
same principle which makes the hen ruffle her feathers and 
expand her wings when a dog approaches her chickens ; but 
I have not space here to enlarge on the many ways by which 
animals endeavour to frighten away their enemies. 

Natural selection will never produce in a being any struc- 
ture more injurious than beneficial to that being, for natural 
selection acts solely by and for the good of each. No organ 
will be formed, as Paley has remarked, for the purpose of 
causing pain or for doing an injury to its possessor. If a fair 
balance be struck between the good and evil caused by each 
part, each will be found on the whole advantageous. After 
the lapse of time, under changing conditions of life, if any 
part comes to be injurious, it will be modified; or if it be not 
so, the being will become extinct as myriads have become 

Natural selection tends only to make each organic being 
as perfect as, or slightly more perfect than, the other inhabi- 
tants of the same country with which it comes into competi- 
tion. And we see that this is the standard of perfection 
attained under nature. The endemic productions of New Zea- 
land, for instance, arc perfect one compared with another; 
but they arc now rapidly yielding before the advancing le- 
gions of plants and animals introduced from Europe. Natural 
selection will not produce absolute perfection, nor do wc 


always meet, as far as we can judge, with this high standard 
under nature. The correction for the aberration of Hght 
is said by Miiller not to be perfect even in that most perfect 
organ, the human eye. Helmholtz, whose judgm.ent no one 
will dispute, after describing in the strongest terms the won- 
derful power of the human eye, adds these remarkable 
words : "That which we have discovered in the way of in- 
exactness and imperfection in the optical machine and in 
the image on the retina, is as nothing in comparison with 
the incongruities which we have just come across in the 
domain of the sensations. One might say that nature has 
taken delight in accumulating contradictions in order to re- 
move all foundations from the theory of a pre-existing har- 
mony between the external and internal worlds." If our 
reason leads us to admire with enthusiasm a multitude of 
inimitable contrivances in nature, this same reason tells us, 
though we may easily err on both sides, that some other con- 
trivances are less perfect. Can we consider the sting of the 
bee as perfect, which, when used against many kinds of 
enemies, cannot be withdrawn, owing to the backward serra- 
tures, and thus inevitably causes the death of the insect by 
tearing out its viscera? 

If we look at the sting of the bee, as having existed in a 
remote progenitor, as a boring and serrated instrument, like 
that in so many members of the same great order, and that 
it has since been modified, but not perfected for its present 
purpose, with the poison originally adapted for some other 
object, such as to produce galls, since intensified, we can per- 
haps understand how it is that the use of the sting should so 
often cause the insect's own death : for if on the whole the 
power of stinging be useful to the social community, it will 
fulfil all the requirements of natural selection, though it 
may cause the death of some few members. If we admire 
the truly wonderful power of scent by which the males of 
many insects find their females, can we admire the produc- 
tion for this single purpose of thousands of drones, which 
are utterly useless to the community for any other purpose, 
and which are ultimately slaughtered by their industrious and 
sterile sisters? It may be difficult, but we ought to admire 
the savage instinctive hatred of the queen-bee, which urges 


her to destroy the yonnpf queens, her daughters, as soon as 
they are born, or to perish herself in the combat; for un- 
doubtedly this is for the good of the community ; and mater- 
nal love or maternal hatred, though the latter fortunately is 
most rare, is all the same to the inexorable principle of 
natural selection. If we admire the several ingenious contri- 
vances, by which orchids and many other plants are fertilised 
through insect agency, can we consider as equally perfect 
the elaboration of dense clouds of pollen by our fir-trees, so 
that a few granules may be wafted by chance on to the ovules? 

summary: the law of unity of type and of the 

conditions of existence embkaced by the 

theory of natural selection 

We have in this chapter discussed some of the difficulties 
and objections which may be urged against the theory. 
Many of them are serious ; but I think that in the discussion 
light has been thrown on several facts, which on the belief 
of independent acts of creation are utterly obscure. We have 
seen that species at any one period are not indefinitely vari- 
able, and are not linked together by a multitude of interme- 
diate gradations, partly because the process of natural selec- 
tion is always very slow, and at any one time acts only on a 
few forms ; and partly because the very process of natural 
selection implies the continual supplanting and extinction 
of preceding and intermediate gradations. Closely allied 
species, now living on a continuous area, must often have 
been formed when the area was not continuous, and when the 
conditions of life did not insensibly graduate away from one 
part to another. When two varieties are formed in two dis- 
tricts of a continuous area, an intermediate variety will often 
be formed, fitted for an intermediate zone ; but from 
reasons assigned, the intermediate variety will usually exist 
in lesser numbers than the two forms which it connects: con- 
sequently the two latter, during the course of further modi- 
fication, from existing in greater numbers, will have a great 
advantage over the less numerous intermediate variety, and 
will thus generally succeed in supplanting and extermi- 
nating it. 


We have seen in this chapter how cautious we should be 
in concluding that the most different habits of life could not 
graduate into each other; that a bat, for instance, could not 
have been formed by natural selection from an animal which 
at first only glided through the air. 

W e have seen that a species under new conditions of life 
may change its habits; or it may have diversified habits, with 
some verj^ unlike those of its nearest congeners. Hence we 
can understand, bearing in mind that each organic being is 
trying to live wherever it can live, how it has arisen that 
there are upland geese with webbed feet, ground woodpeck- 
ers, diving thrushes, and petrels with the habits of auks. 

Although the behef that an organ so perfect as the eye 
could have been formed by natural selection, is enough to 
stagger any one; yet in the case of any organ, if we know of 
a long series of gradations in complexit}-, each good for its 
possessor, then, under changing conditions of life, there is 
no logical impossibiHty in the acquirement of any conceivable 
degree of perfection through natural selection. In the cases 
in which we know of no intermediate or transitional states, 
we should be extremely cautious in concluding that none can 
have existed, for the metamorphoses of many organs show 
what wonderful changes in function are at least possible. 
For instance, a swimbladder has apparently been converted 
into an air-breathing lung. The same organ having per- 
formed simultaneously very different functions, and then 
having been in part or in whole specialised for one function ; 
and two distinct organs having performed at the same time 
the same function, the one having been perfected whilst aided 
by the other, must often have largely facilitated transitions. 

We have seen that in two beings widely remote from each 
other in the natural scale, organs serving for the same pur- 
pose and in external appearance closely similar may have 
been separately and independently formed: but when such 
organs are closely examined, essential differences in their 
structure can almost always be detected; and this naturally 
follows from the principle of natural selection. On the 
other hand, the common rule throughout nature is infinite 
diversity of structure for gaining the same end; and this 
again naturally follows from the same great principle. 


In many cases we are far too ignorant to be enabled to 
assert that a part or organ is so unimportant for the welfare 
of a species, that modifications in its structure could not 
have been slowly accumulated by means of natural selection. 
In many other cases, modifications are probably the direct 
result of the laws of variation or of growth, independently 
of any good having been thus gained. But even such struc- 
tures have often, as we may feel assured, been subsequently 
taken advantage of, and still further modified, for the good 
of species under new conditions of life. We may, also, be- 
lieve that a part formerly of high importance has frequently 
been retained (as the tail of an aquatic animal by its terres- 
trial descendants), though it has become of such small im- 
portance that it could not, in its present state, have been 
acquired by means of natural selection. 

Natural selection can produce nothing in one species for 
the exclusive good or injury of another ; though it may well 
produce parts, organs, and excretions highly useful or even 
indispensable, or again highly injurious to another species, 
but in all cases at the same time useful to the possessor. In 
each well-stocked country natural selection acts through the 
competition of the inhabitants, and consequently leads to suc- 
cess in the battle for life, only in accordance with the 
standard of that particular country. Hence the inhabitants 
of one country, generally the smaller one, often yield to the 
inhabitants of another and generally the larger country. 
For in the larger country there will have existed more indi- 
viduals and more diversified forms, and the competition will 
have been severer, and thus the standard of perfection will 
have been rendered higher. Natural selection will not neces- 
sarily lead to absolute perfection ; nor, as far as we can judge 
by our limited faculties, can absolute perfection be every- 
where predicated. 

On the theory of natural selection we can clearly under- 
stand the full meaning of that old canon in natural history, 
"Natura non facit saltum." This canon, if we look to the 
present inhabitants alone of the world, is not strictly cor- 
rect; but if we include all those of past times, whether known 
or unknown, it must on this theory be strictly true. 

It is generally acknowledged that all organic beings have 


been formed on two great laws — Unity of Type, and the 
Conditions of Existence. By unity of type is meant that 
fundamental agreement in structure which we see in organic 
beings of the same class, and which is quite independent of 
their habits of life. On my theory, unity of type is explained 
by unity of descent. The expression of conditions of exist- 
ence, so often insisted on by the illustrious Cuvier, is fully 
embraced by the principle of natural selection. For natural 
selection acts by either now adapting the varying parts of 
each being to its organic and inorganic conditions of life; 
or by having adapted them during past periods of time : the 
adaptations being aided in many cases by the increased use 
or disuse of parts, being affected by the direct action of the 
external conditions of life, and subjected in all cases to the 
several laws of growth and variation. Hence, in fact, the 
law of the Conditions of Existence is the higher law; as it 
includes, through the inheritance of former variations and 
adaptations, that of Unity of Type. 


Miscellaneous Objections to the Theory of Natural 


Longevity — Modifications not necessarily simultaneous — Modifications 
apparently of no direct service — Progressive development — 
Characters of small functional importance, the most constant — 
Supposed incompetence of natural selection to account for the 
incipient stages of useful -structures — Causes which interfere with 
the acquisition through natural selection of useful structures — 
Gradations of structure with changed functions — Widely different 
organs in members of the same class, developed from one and 
the same source — Reasons for disbelieving in great and abrupt 

I WILL devote this chapter to the consideration of various 
miscellaneous objections which have been advanced 
against my views, as some of the previous discus- 
sions may thus be made clearer; but it would be useless 
to discuss all of them, as many have been made by writers 
who have not taken the trouble to understand the subject. 
Thus a distinguished German naturalist has asserted that 
the weakest part of my theory is, that I consider all organic 
beings as imperfect: what I have really said is, that all are 
not as perfect as they might have been in relation to their 
conditions; and this is shown to be the case by so many 
native forms in many quarters of the world having yielded 
their places to intruding foreigners. Nor can organic beings, 
even if they were at any one time perfectly adapted to their 
conditions of life, have remained so, when their conditions 
changed, unless they themselves likewise changed; and no 
one will dispute that the physical conditions of each country, 
as well as the numbers and kinds of its inhabitants, have 
undergone many mutations. 

A critic has lately insisted, with some parade of mathe- 
matical accuracy, that longevity is a great advantage to all 
species, so that he who believes in natural selection "must 



arrange his genealogical tree" in such a manner that all the 
descendants have longer lives than their progenitors! Can- 
not our critic conceive that a biennial plant or one of the 
lower animals might range into a cold climate and perish 
there every winter; and yet, owing to advantages gained 
through natural selection, survive from year to year, by 
means of its seeds or ova? Mr. E. Ray Lankester has re- 
cently discussed this subject, and he concludes, as far as its 
extreme complexity allows him to form a judgment, that 
longevity is generally related to the standard of each species 
in the scale of organisation, as well as to the amount of ex- 
penditure in reproduction and in general activity. And these 
conditions have, it is probable, been largely determined 
through natural selection. 

It has been argued that, as none of the animals and plants 
of Egypt, of which we know anything, have changed during 
the last three or four thousand years, so probably have none 
in any part of the world. But, as Mr. G. H. Lewes has re- 
marked, this line of argument proves too much, for the 
ancient domestic races figured on the Egyptian monuments, 
or embalmed, are closely similar or even identical with those 
now living; yet all naturalists admit that such races have 
been produced through the modification of their original 
types. The many animals which have remained unchanged 
since the commencement of the glacial period, would have 
been an incomparably stronger case, for these have been 
exposed to great changes of climate and have migrated over 
great distances; whereas, in Egypt, during the last several 
thousand years, the conditions of life, as far as we know, have 
remained absolutely uniform. The fact of little or no modifi- 
cation having been effected since the glacial period would have 
been of some avail against those who believe in an innate and 
necessary law of development, but is powerless against the 
doctrine of natural selection or the survival of the fittest, 
which implies that when variations or individual differences 
of a beneficial nature happen to arise, these will be preserved; 
but this will be effected only under certain favourable cir- 

The celebrated palaeontologist, Bronn, at the close of hisi 
German translation of this work, asks, how, on the principle 



of natural selection, can a variety live side by side with the 
parent species? If both have become fitted for slightly dif- 
ferent habits of life or conditions, they might live together; 
and if we lay on one side polymoi-phic species, in which the 
variability seems to be of a peculiar nature, and all mere 
temporary variations, such as size, albinism, &c., the more 
permanent varieties are generally found, as far as I can 
discover, inhabiting distinct stations, — such as high land or 
low land, dry or moist districts. Moreover, in the case of 
animals which wander much alx)Ut and cross freely, their 
varieties seem to be generally confined to distinct regions. 

Bronn also insists that distinct species never differ from 
each other in single characters, but in many parts ; and he 
asks, how it always comes that many parts of the organisa- 
tion should have been modified at the same time through 
variation and natural selection? But there is no necessity 
for supposing that all the parts of any being have been 
simultaneously modified. The most striking modifications, 
excellently adapted for some purpose, might, as was formerly 
remarked, be acquired by successive variations, if slight, 
first in one part and then in another; and as they would be 
transmitted all together, they would appear to us as if they 
had been simultaneously developed. The best answer, how- 
ever, to the above objection is afforded by those domestic 
races which have been modified, chiefly through man's power 
of selection, for some special purpose. Look at the race 
and dray horse, or at the grey-hound and mastiff. Their 
whole frames and even their mental characteristics have been 
modified; but if we could trace each step in the history of 
their transformation,— and the latter steps can be traced,— 
we should not see great and simultaneous changes, but first 
one part and then another slightly modified and improved. 
Even when selection has been applied by man to some one 
character alone, — of which our cultivated plants offer the 
best instances, — it will invariably be found that although 
this one part, whether it be the flower, fruit, or leaves, has 
been greatly changed, almost all the other parts have been 
slightly modified. This may be attributed partly to the prin- 
ciple of correlated growth, and partly to so-called spon- 
taneous variation. 


A much more serious objection has been urged by Bronn, 
and recently by Broca, namely, that many characters appear 
to be of no service whatever to their possessors, and therefore 
cannot have been influenced through natural selection. 
Bronn adduces the length of the ears and tails in the dif- 
ferent species of hares and mice, — the complex folds of 
enamel in the teeth of many animals, and a multitude of 
analogous cases. With respect to plants, this subject has 
been discussed by Nageli in an admirable essay. He admits 
that natural selection has effected much, but he insists that 
the families of plants differ chiefly from each other in mor- 
phological characters, which appear to be quite unimportant 
for the welfare of the species. He consequently believes in 
an innate tendency towards progressive and more perfect 
development. He specifies the arrangement of the cells in 
the tissues, and of the leaves on the axis, as cases in which 
natural selection could not have acted. To these may be 
added the numerical divisions in the parts of the flower, the 
position of the ovules, the shape of the seed, when not of any 
use for dissemination, &c. 

There is much force in the above objection. Nevertheless, 
we ought, in the first place, to be extremely cautious in 
pretending to decide what structures now are, or have for- 
merly been, of use to each species. In the second place, it 
should always be borne in mind that when one part is modi- 
fied, so will be other parts, through certain dimly seen causes, 
such as an increased or diminished flow of nutriment to a 
part, mutual pressure, an early developed part affecting one 
subsequently developed, and so forth, — as well as through 
other causes which lead to the many mysterious cases of 
correlation, which we do not in the least understand. These 
agencies may be all grouped together, for the sake of brevity, 
under the expression of the laws of growth. In the third 
place, we have to allow for the direct and definite action of 
changed conditions of life, and for so-called spontaneous 
variations, in which the nature of the conditions apparently 
plays a quite subordinate part. Bud-variations, such as the 
appearance of a moss-rose on a common rose, or of a nec- 
tarine on a peach-tree, offer good instances of spontaneous 
variations; but even in these cases, if we bear in mind the 


power of a minute drop of poison in producing complex galls, 
we ought not to feel too sure that the above variations are 
not the effect of some local change in the nature of the sap, 
due to some change in the conditions. There must be some 
efficient cause for each slight individual difference, as well 
as for more strongly marked variations which occasionally 
arise; and if the unknown cause were to act persistently, it 
is almost certain that all the individuals of the species would 
be similarly modified. 

In the earlier editions of this work I under-rated, as it now 
seems probable, the frequency and importance of modifica- 
tions due to spontaneous variability. But it is impossible to 
attribute to this cause the innumerable structures which are 
so well adapted to the habits of life of each species. I can 
no more believe in this, than that the well-adapted form of a 
race-horse or greyhound, which before the principle of selec- 
tion by man was well understood, excited so much surprise in 
the minds of the older naturalists, can thus be explained. 

It may be worth while to illustrate some of the foregoing 
remarks. With respect to the assumed inutility of various 
parts and organs, it is hardly necessary to observe that even 
in the higher and best-known animals many structures exist, 
which are so highly developed that no one doubts that they 
are of importance, yet their use has not been, or has only 
recently been, ascertained. As Bronn gives the length of 
the ears and tail in the several species of mice as instances, 
though trifling ones, of differences in structure which can 
be of no special use, I may mention that, according to Dr. 
Schobl, the external ears of the common mouse are supplied 
in an extraordinary manner with nerves, so that they no 
doubt serve as tactile organs ; hence the length of the ears 
can hardly be quite unimportant. We shall, also, presently 
see that the tail is a highly useful prehensile organ to some 
of the species ; and its use would be much influenced by its 

With respect to plants, to which on account of Nageli's 
essay I shall confine myself in the following remarks, it will 
be admitted that the flowers of orchids present a multitude of 
curious structures, which a few years ago would have been 
considered as mere morphological differences without any 


special function ; but they are now known to be of the highest 
importance for the fertilisation of the species through the 
aid of insects, and have probably been gained through natural 
selection. No one until lately would have imagined that in 
dimorphic and trimorphic plants the different lengths of the 
stamens and pistils, and their arrangement, could have been 
of any service, but now we know this to be the case. 

In certain whole groups of plants the ovules stand erect, 
and in others they are suspended; and within the same 
ovarium of some few plants, one ovule holds the former and 
a second ovule the latter position. These positions seem at 
first purely morphological, or of no physiological significa- 
tion; but Dr. Hooker informs me that within the same 
ovarium, the upper ovules alone in some cases, and in other 
cases the lower ones alone are fertilised ; and he suggests that 
this probably depends on the direction in which the pollen- 
tubes enter the ovarium. If so, the position of the ovules, 
even when one is erect and the other suspended within the 
same ovarium, would follow from the selection of any slight 
deviations in position which favoured their fertihsation, and 
the production of seed. 

Several plants belonging to distinct orders habitually pro- 
duce flowers of two kinds, — the one open of the ordinary 
structure, the other closed and imperfect. These two kinds 
of flowers sometimes differ wonderfully in structure, yet may 
be seen to graduate into each other on the same plant. The 
ordinary and open flowers can be intercrossed ; and the bene- 
fits which certainly are derived from this process are thus 
secured. The closed and imperfect flowers are, however, 
manifestly of high importance, as they yield with the utmost 
safety a large stock of seed, with the expenditure of won- 
derfully little pollen. The two kinds of flowers often differ 
much, as just stated, in structure. The petals in the imperfect 
flowers almost always consist of mere rudiments, and the 
pollen-grains are reduced in diameter. In Ononis columnse 
five of the alternate stamens are rudimentary; and in some 
species of Viola three stamens are in this state, two retaining 
their proper function, but being of very small size. In six 
out of thirty of the closed flowers in an Indian violet (name 
unknown, for the plants have never produced with me per^ 


feet flowers), the sepals are reduced from the normal num- 
ber of five to three. In one section of the Malpighiacca; the 
closed flowers, according to A. de Jussieu, are still further 
modified, for the five stamens which stand opposite to the 
sepals are all aborted, a sixth stamen standing opposite to 
a petal being alone developed ; and this stamen is not present 
in the ordinary flowers of these species; the style is aborted; 
and the ovaria are reduced from three to two. Now although 
natural selection may well have had the power to prevent 
some of the flowers from expanding, and to reduce the amount 
of pollen, when rendered by the closure of the flowers super- 
fluous, yet hardly any of the above special modifications can 
have been thus determined, but must have followed from 
the laws of growth, including the functional inactivity of 
parts, during the progress of the reduction of the pollen and 
the closure of the flowers. 

It is so necessary to appreciate the important effects of 
the laws of growth, that I will give some additional cases of 
another kind, namely of differences in the same part or organ, 
due to differences in relative position on the same plant. 
In the Spanish chestnut, and in certain fir-trees, the angles of 
divergence of the leaves differ, according to Schacht, in 
the nearly horizontal and in the upright branches. In the 
common rue and some other plants, one flower, usually the 
central or terminal one, opens first, and has five sepals and 
petals, and five divisions to the ovarium ; whilst all the other 
flowers on the plant are tetramerous. In the British Adoxa 
the uppermost flower generally has two calyx-lobes with the 
other organs tetramerous, whilst the surrounding flowers 
generally have three calyx-lobes with the other organs pen- 
tamerous. In many Composite and Umbelliferae (and in 
some other plants) the circumferential flowers have their 
corollas much more developed than those of the centre; 
and this seems often connected with the abortion of the re- 
productive organs. It is a more curious fact, previously 
referred to, that the achenes or seeds of the circumference 
and centre sometimes differ greatly in form, colour, and 
other characters. In Carthamus and some other Compositre 
the central achenes alone are furnished with a pappus ; and 
in Hyoseris the same head yields achenes of three different 

II— [[(• XI 


forms. In certain Umbelliferae the exterior seeds, according 
to Tausch, are orthospermous, and the central one coelosper- 
mous, and this is a character which was considered by De 
Candolle to be in other species of the highest systematic im- 
portance. Prof. Braun mentions a Fumariaceous genus in 
which the flowers in the lower part of the spike bear oval, 
ribbed, one-seeded nutlets ; and in the upper part of the spike, 
lanceolate, two-valved, and two-seeded siliques. In these 
several cases, with the exception of that of the well developed 
ray-florets, which are of service in making the flowers con- 
spicuous to insects, natural selection cannot, as far as we can 
judge, have come into play, or only in a quite subordinate 
manner. All these modifications follow from the relative 
position and inter-action of the parts ; and it can hardly be 
doubted that if all the flowers and leaves on the same plant 
had been subjected to the same external and internal con- 
dition, as are the flowers and leaves in certain positions, all 
would have been modified in the same manner. 

In numerous other cases we find modifications of structure, 
which are considered by botanists to be generally of a highly 
important nature, affecting only some of the flowers on the 
same plant, or occurring on distinct plants, which grow close 
together under the same conditions. As these variations 
seem of no special use to the plants, they cannot have been 
influenced by natural selection. Of their cause we are quite 
ignorant ; we cannot even attribute them, as in the last class 
of cases, to any proximate agency, such as relative position. 
I will give only a few instances. It is so common to observe 
on the same plant, flowers indifferently tetramerous, pentam- 
erous, &c., that I need not give examples ; but as numerical 
variations are comparatively rare when the parts are few, I 
may mention that, according to De Candolle, the flowers of 
Papaver bracteatum offer either two sepals with four petals 
(which is the common type with poppies), or three sepals 
with six petals. The manner in which the petals are folded 
in the bud is in most groups a very constant morphological 
character ; but Professor Asa Gray states that with some 
species of Mimulus, the aestivation is almost as frequently 
that of the Rhinanthidese as of the Antirrhinide?e, to which 
latter tribe the genus belongs. Aug. St. Hilaire gives the 


following cases : the genus Zanthoxylon belongs to a division 
of the Rutacese with a single ovary, but in some species 
flowers may be found on the same plant, and even in the 
same panicle, with either one or two ovaries. In Helian- 
themum the capsule has been described as unilocular or 
3-locular; and in H. mutabile, "Une lame, plus on mains 
large, s'etend entre le pericarpe et le placenta." In the 
flowers of Saponaria officinalis, Dr. Masters has observed 
instances of both marginal and free central placentation. 
Lastly, St. Hilaire found towards the southern extreme of 
the range of Gomphia olea^formis two forms which he did 
not at first doubt were distinct species, but he subsequently 
saw them growing on the same bush; and he then adds, 
"Voila done dans un meme individu des loges et un style qui 
se rattachent tantot a un axe verticale et tantot a un 

We thus see that with plants many morphological changes 
may be attributed to the laws of growth and the inter-action 
of parts, independently of natural selection. But with re- 
spect to Nageli's doctrine of an innate tendency towards 
perfection or progressive development, can it be said in the 
case of these strongly pronounced variations, that the plants 
have been caught in the act of progressing towards a higher 
state of development ? On the contrary, I should infer from 
the mere fact of the parts in question differing or varying 
greatly on the same plant, that such modifications were of 
extremely small importance to the plants themselves, of 
whatever importance they may generally be to us for our clas- 
sifications. The acquisition of a useless part can hardly be said 
to raise an organism in the natural scale ; and in the case of the 
imperfect, closed flowers above described, if any new principle 
has to be invoked, it must be one of retrogression rather than 
of progression; and so it must be with many parasitic and 
degraded animals. We are ignorant of the exciting cause of 
the above specified modifications; but if the unknown cause 
were to act almost uniformly for a length of time, we may 
infer that the result would be almost uniform ; and in this 
case all the individuals of the species would be modified in 
the same manner. 

From the fact of the above characters being unimportant 


'for the welfare of the species, any slight variations which oc- 
curred in them would not have been accumulated and aug- 
mented through natural selection. A structure which has been 
developed through long-continued selection, when it ceases to 
be of service to a species, generally becomes variable, as we 
see with rudimentary organs ; for it will no longer be regu- 
lated by this same power of selection. But when, from the 
nature of the organism and of the conditions, modifications 
have been induced which are unimportant for the welfare of 
the species, they may be, and apparently often have been, 
transmitted in nearly the same state to numerous, otherwise 
modified, descendants. It cannot have been of much impor- 
tance to the greater number of mammals, birds, or reptiles, 
whether they were clothed with hair, feathers, or scales ; yet 
hair has been transmitted to almost all mammals, feathers 
to all birds, and scales to all true reptiles. A structure, what- 
ever it may be, which is common to many allied forms, is 
ranked by us as of high systematic importance, and conse- 
quently is often assumed to be of high vital importance to the 
species. Thus, as I am inclined to believe, morphological 
differences, which we consider as important — such as the ar- 
rangement of the leaves, the divisions of the flower or of the 
ovarium, the position of the ovules, &c. — first appeared in 
many cases as fluctuating variations, which sooner or later 
became constant through the nature of the organism and of 
the surrounding conditions, as well as through the inter- 
crossing of distinct individuals, but not through natural selec- 
tion; for as these morphological characters do not affect the 
welfare of the species, any slight deviations in them could 
not have been governed or accumulated through this latter 
agency. It is a strange result which we thus arrive at, 
namely that characters of slight vital importance to the spe- 
cies, are the most important to the systematist; but, as we 
shall hereafter see when we treat of the genetic principle of 
classification, this is by no means so paradoxical as it may 
at first appear. 

Although we have no good evidence of the existence in 
organic beings of an innate tendency towards progressive 
development, yet this necessarily follows, as I have attempted 
to show in the fourth chapter, through the continued action 


of natural selection. For the best definition which has ever 
been given of a high standard of organisation is the degree 
to which the parts have been specialised or differentiated; 
and natural selection tends towards this end, inasmuch as 
the parts are thus enabled to perform their functions more 

A distinguished zoologist, !Mr. St. George Mivart, has 
recently collected all the objections which have ever been 
advanced by myself and others against the theory of natural 
selection, as propounded by Mr. Wallace and myself, and has 
illustrated them with admirable art and force. When thus 
marshalled, they make a formidable array ; and as it forms 
no part of Mr. Mivart's plan to give the various facts and 
considerations opposed to his conclusions, no slight effort 
of reason and memory is left to the reader, who may wish 
to weigh the evidence on both sides. When discussing special 
cases, Mr. Mivart passes over the effects of the increased 
use and disuse of parts, which I have always maintained to 
be highly important, and have treated in my 'Variation under 
Domestication' at greater length than, as I believe, any other 
writer. He likewise often assumes that I attribute nothing 
to variation, independently of natural selection, whereas in 
the work just referred to I have collected a greater number of 
well-established cases than can be found in any other work 
known to me. My judgment may not be trustworthy, but 
after reading with care Mr. Mivart's book, and comparing 
each section with what I have said on the same head, I never 
before felt so strongly convinced of the general truth of the 
conclusions here arrived at, subject, of course, in so intricate 
a subject, to much partial error. 

All Mr. Mivart's objections will be, or have been, con- 
sidered in the present volume. The one new point which 
appears to have struck many readers is, "that natural selec- 
tion is incompetent to account for the incipient stages of 
useful structures." This subject is intimately connected with 
that of the gradation of characters, often accompanied by 
a change of function, — for instance, the conversion ^ of a 
swim-bladder into lungs, — points which were discussed in the 
last chapter under two headings. Nevertheless, I will here 


consider in some detail several of the cases advancecl by Mt. 
Mivart, selecting those which are the most illustrative, as 
want of space prevents me from considering all. 

The giraffe, by its lofty stature^ much elongated neck, 
fore legs, head and tongue, has its whole frame beautifully 
adapted for browsing on the higher branches of trees. It can 
thus obtain food beyond the reach of the other Ungulata or 
hoofed animals inhabiting the same country ; and this must 
be a great advantage to it during dearths. The Niata cattle 
in S. America show us how small a difference in structure 
may make, during such periods, a great difference in preserv- 
ing an animal's life. These cattle can browse as well as 
others on grass, but from the projection of the lower jaw 
they cannot, during the often recurrent droughts, browse on 
the twigs of trees, reeds, &c., to which food the common 
cattle and horses are then driven ; so that at these times the 
Niatas perish, if not fed by their owners. Before coming 
to Mr, Alivart's objections, it may be well to explain once 
again how natural selection will act in all ordinary cases. 
Man has modified some of his animals, without necessarily 
having attended to special points of structure, by simply pre- 
serving and breeding from the fleetest individuals, as with 
the race-horse and greyhound, or as with the game-cock, by 
breeding from the victorious birds. So under nature with 
the nascent giraffe, the individuals which were the highest 
browsers and were able during dearths to reach even an inch 
or two above the others, will often have been preserved; 
for they will have roamed over the whole country in search 
of food. That the individuals of the same species often 
differ slightly in the relative lengths of all their parts may 
be seen in many works of natural history, in which careful 
measurements are given. These slight proportional differ- 
ences, due to the laws of growth and variation, are not of the 
slightest use or importance to most species. But it will have 
been otherwise with the nascent giraffe, considering its prob- 
able habits of life ; for those individuals which had some one 
part or several parts of their bodies rather more elongated 
than usual, would generally have survived. These will have 
intercrossed and left offspring, either inheriting the same 
bodily peculiarities, or with a tendency to vary again in the 


same manner; whilst the individuals, less favoured in the 
same respects, will have been the most liable to perish. 

We here see that there is no need to separate single pairs, 
as man does, when he methodically improves a breed ; natural 
selection will preserve and thus separate all the superior 
individuals, allowing them freely to intercross, and will de- 
stroy all the inferior individuals. By this process long- 
continued, which exactly corresponds with what I have called 
unconscious selection by man, combined no doubt in a most 
important manner with the inherited effects of the increased 
use of parts, it seems to me almost certain that an ordinary 
hoofed quadruped might be converted into a giraffe. 

To this conclusion Mr. Mivart brings forward two objec- 
tions. One is that the increased size of the body would 
obviously require an increased supply of food, and he con- 
siders it as "very problematical whether the disadvantages 
thence arising would not, in times of scarcity, more than 
counterbalance the advantages." But as the giraffe does 
actually exist in large numbers in S. Africa, and as some of 
the largest antelopes in the world, taller than an ox, abound 
there, why should we doubt that, as far as size is concerned, 
intermediate gradations could formerly have existed there, 
subjected as now to severe dearths? Assuredly the being 
able to reach, at each stage of increased size, to a supply of 
food, left untouched by the other hoofed quadrupeds of the 
country, would have been of some advantage to the nascent 
giraffe. Nor must we overlook the fact, that increased bulk 
would act as a protection against almost all beasts of prey 
excepting the lion ; and against this animal, its tall neck, — 
and the taller the better, — would, as Mr. Chauncey Wright 
has remarked, serve as a watch-tower. It is from this cause, 
as Sir S. Baker remarks, that no animal is more difficult to 
stalk than the giraffe. This animal also uses its long neck 
as a means of offence or defence, by violently swinging its 
head armed with stump-like horns. The preservation of 
each species can rarely be determined by any one advantage 
but by the union of all, great and small. 

Mr. Mivart then asks (and this is his second objection), 
if natural selection be so potent, and if high browsing be so 
great an advantage, why has not any other hoofed quadruped 


acquired a long neck' and lofty stature, besides the giraffe, 
and, in lesser degree, the camel, guanaco, and macrauchenia ? 
Or, again, why has not any member of the group acquired a 
long proboscis? With respect to S. Africa, which was for- 
merly inhabited by numerous herds of the giraffe, the answer 
is not difficult, and can best be given by an illustration. In 
every meadow in England in which trees grow, we see the 
lower branches trimmed or planed to an exact level by the 
browsing of the horses or cattle ; and what advantage would 
it be, for instance, to sheep, if kept there, to acquire slightly 
longer necks? In every district some one kind of animal 
will almost certainly be able to browse higher than the 
others; and it is almost equally certain that this one kind 
alone could have its neck elongated for this purpose, through 
natural selection and the effects of increased use. In S. 
Africa the competition for browsing on the higher branches 
of the acacias and other trees must be between giraffe and 
giraffe, and not with the other ungulate animals. 

Why, in other quarters of the world, various animals be- 
longing to this same order have not acquired either an 
elongated neck or a proboscis, cannot be distinctly answered; 
but it is as unreasonable to expect a distinct answer to such 
a question, as why some event in the history of mankind did 
not occur in one country, whilst it did in another. We are 
ignorant with respect to the conditions which determine the 
numbers and range of each species ; and we cannot even con- 
jecture what changes of structure would be favourable to 
its increase in some new country. We can, however, see in a 
general manner that various causes might have interfered 
with the development of a long neck or proboscis. To reach 
the foliage of a considerable height (without climbing, for 
which hoofed animals are singularly ill-constructed) implies 
greatly increased bulk of body ; and we know that some areas 
support singxilarly few large quadrupeds, for instance S. 
America, though it is so luxuriant; whilst S. Africa abounds 
with them to an unparalleled degree. Why this should be 
so, we do not know; nor why the later tertiary periods should 
have been much more favourable for their existence than the 
present time. Whatever the causes may have been, we can 
see that certain districts and times would have been much 


more favourable than others for the development of so large 
a quadruped as the giraffe. 

In order that an animal should acquire some structure 
specially and largely developed, it is almost indispensable 
that several other parts should be modified and co-adapted. 
Although every part of the body varies slightly, it does not 
follow that the necessary parts should always vary in the 
right direction and to the right degree. With the different 
species of our domesticated animals we know that the parts 
vary in a different manner and degree; and that some species 
are much more variable than others. Even if the fitting vari- 
ations did arise, it does not follow that natural selection 
would be able to act on them, and produce a structure which 
apparently would be beneficial to the species. For instance, 
if the number of individuals existing in a country is deter- 
mined chiefly through destruction by beasts of prey, — by ex- 
ternal or internal parasites, etc., — as seems often to be the 
case, then natural selection will be able to do little, or will be 
greatly retarded, in modifying any particular structure for ob- 
taining food. Lastly, natural selection is a slow process, and 
the same favourable conditions must long endure in order 
that any marked effect should thus be produced. Except by 
assigning such general and vague reasons, we cannot explain 
why, in many quarters of the world, hoofed quadrupeds have 
not acquired much elongated necks or other means for brows- 
ing on the higher branches of trees. 

Objections of the same nature as the foregoing have been 
advanced by many writers. In each case various causes, be- 
sides the general ones just indicated, have probably inter- 
fered with the acquisition through natural selection of struc- 
tures, which it is thought would be beneficial to certain 
species. One writer asks, why has not the ostrich acquired 
the power of flight? But a moment's reflection will show 
what an enormous supply of food would be necessary to give 
to this bird of the desert force to move its huge body through 
the air. Oceanic islands are inhabited by bats and seals, but 
by no terrestrial mammals ; yet as some of these bats are 
peculiar species, they must have long inhabited their present 
homes. Therefore Sir C. Lyell asks, and assigns certain rea- 
sons in answer, why have not seals and bats given birth on 


such islands to forms fitted to live on the land? But seals 
would necessarily be first converted into terrestrial carnivor- 
ous animals of considerable size, and bats into terrestrial 
insectivorous animals; for the former there would be no 
prey; for the bats ground-insects would serve as food, but 
these would already be largely preyed on by the reptiles or 
birds, which first colonise and abound on most oceanic islands. 
Gradations of structure, with each stage beneficial to a chang- 
ing species, will be favoured only under certain peculiar con- 
ditions. A strictly terrestrial animal, by occasionally hunting 
for food in shallow water, then in streams or lakes, might at 
last be converted into an animal so thoroughly aquatic as to 
brave the open ocean. But seals would not find on oceanic 
islands the conditions favourable to their gradual reconver- 
sion into a terrestrial form. Bats, as formerly shown, prob- 
ably acquired their wings by at first gliding through the air 
from tree to tree, like the so-called flying squirrels, for the 
sake of escaping from their enemies, or for avoiding falls; 
but when the power of true flight had once been acquired, it 
would never be reconverted back, at least for the above pur- 
poses, into the less efficient power of gliding through the air. 
Bats might, indeed, like many birds, have had their wings 
greatly reduced in size, or completely lost, through disuse; 
but in this case it would be necessary that they should first 
have acquired the power of running quickly on the ground, 
by the aid of their hind legs alone, so as to compete with 
birds or other ground animals; and for such a change a bat 
seems singularly ill-fitted. These conjectural remarks have 
been made merely to show that a transition of structure, with 
each step beneficial, is a highly complex affair ; and that there 
is nothing strange in a transition not having occurred in any 
particular case. 

Lastly, more than one writer has asked, why have some 
animals had their mental powers more highly developed than 
others, as such development would be advantageous to all? 
Why have not apes acquired the intellectual powers of man ? 
Various causes could be assigned; but as they are conjec- 
tural, and their relative probability cannot be weighed, it 
would be useless to give them. A definite answer to the lat- 
ter question ought not to be expected, seeing that no one can 


solve the simpler problem why, of two races of savages, one 
has risen higher in the scale of civilisation than the other; 
and this apparently implies increased brain-power. 

We will return to Mr. Mivart's other objections. Insects 
often resemble for the sake of protection various objects, such 
as green or decayed leaves, dead twigs, bits of lichen, flowers, 
spines, excrement of birds, and living insects ; but to this lat- 
ter point I shall hereafter recur. The resemblance is often 
wonderfully close, and is not confined to colour, but extends 
to form, and even to the manner in which the insects hold 
themselves. The caterpillars which project motionless like 
dead twigs from the bushes on which they feed, offer an ex- 
cellent instance of a resemblance of this kind. The cases of 
the imitation of such objects as the excrement of birds, are 
rare and exceptional. On this head, Mr. Mivart remarks, 
"As, according to Mr. Darwin's theory, there is a constant 
tendency to indefinite variation, and as the minute incipient 
variations will be in all directions, they must tend to neutral- 
ise each other, and at first to form such unstable modifications 
that it is difficult, if not impossible, to see how such indefinite 
oscillations of infinitesimal beginnings can ever build up a 
sufficiently appreciable resemblance to a leaf, bamboo, or other 
object, for Natural Selection to seize upon and perpetuate." 

But in all the foregoing cases the insects in their original 
state no doubt presented some rude and accidental resem- 
blance to an object commonly found in the stations frequented 
by them. Nor is this at all improbable, considering the al- 
most infinite number of surrounding objects and the diver- 
sity in form and colour of the hosts of insects which exist. 
As some rude resemblance is necessary for the first start, we 
can understand how it is that the larger and higher animals 
do not (with the exception, as far as I know, of one fish") 
resemble for the sake of protection special objects, but only 
the surface which commonly surrounds them, and this chiefly 
in colour. Assuming that an insect originally happened to 
resemble in some degree a dead twig or a decayed leaf, and 
that it varied slightly in many ways, then all the variations 
which rendered the insect at all more like any such object, 
and thus favoured its escape, would be preserved, whilst other 
variations would be neglected and ultimately lost; or, if they 


rendered the insect at all less like the imitated object, they 
would be eliminated. There would indeed be force in Mr. 
Mivart's objection, if we were to attempt to account for the 
above resemblances, independently of natural selection, 
through mere fluctuating variability; but as the case stands 
there is none. 

Nor can I see any force in Mr. Mivart's difficulty with re- 
spect to "the last touches of perfection in the mimicry;" as 
in the case given by Mr. Wallace, of a walking-stick insect 
(Ceroxylus laceratus), which resembles "a stick grown over 
by a creeping moss or jungermannia." So close was this 
resemblance, that a native Dyak maintained that the foli- 
aceous excrescences were really moss. Insects are preyed on 
by birds and other enemies, whose sight is probably sharper 
than ours, and every grade in resemblance which aided an 
insect to escape notice or detection, would tend towards its 
preservation; and the more perfect the resemblance so much 
the better for the insect. Considering the nature of the dif- 
ferences between the species in the group which includes the 
above Ceroxylus, there is nothing improbable in this insect 
having varied in the irregularities on its surface, and in these 
having become more or less green-coloured; for in every 
group the characters which differ in the several species are 
the most apt to vary, whilst the generic characters, or those 
common to all the species, are the most constant. 

The Greenland whale is one of the most wonderful animals 
in the world, and the baleen, or whale-bone, one of its great- 
est peculiarities. The baleen consists of a row, on each side, 
of the upper jaw, of about 300 plates or laminae, which stand 
close together transversely to the longer axis of the mouth. 
Within the main row there are some subsidiary rows. The 
extremities and inner margins of all the plates are frayed 
into stiff bristles, which clothe the whole gigantic palate, and 
serve to strain or sift the water, and thus to secure the 
minute prey on which these great animals subsist. The 
middle and longest lamina in the Greenland whale is ten, 
twelve, or even fifteen feet in length ; but in the different 
species of Cetaceans there are gradations in length ; the 
middle lamina being in one species, according to Scoresby, 


four feet, in another three, in another eighteen inches, and in 
the Balaenoptera rostrata only about nine inches in length. 
The quality of the whale-bone also differs in the different 

With respect to the baleen, Mr. Mivart remarks that if it 
"had once attained such a size and development as to be at 
all useful, then its preservation and augmentation within 
serviceable limits would be promoted by natural selection 
alone. But how to obtain the beginning of such useful de- 
velopment?" In answer, it may be asked, why should not 
the early progenitors of the whales with baleen have pos- 
sessed a mouth constructed something like the lamellated 
beak of a duck? Ducks, like whales, subsist by sifting the 
mud and water; and the family has sometimes been called 
Criblatores, or sifters. I hope that I may not be miscon- 
strued into saying that the progenitors of whales did actually 
possess mouths lamellated like the beak of a duck. I wish 
only to show that this is not incredible, and that the immense 
plates of baleen in the Greenland whale might have been 
developed from such lamellae by finely graduated steps, each 
of service to its possessor. 

The beak of a shoveller-duck (Spatula clypeata) is a more 
beautiful and complex structure than the mouth of a whale. 
The upper mandible is furnished on each side (in the speci- 
men examined by me) with a row or comb formed of i88 
thin, elastic lamellae, obliquely bevelled so as to be pointed, 
and placed transversely to the longer axis of the mouth. 
They arise from the palate, and are attached by flexible mem- 
brane to the sides of the mandible. Those standing towards 
the middle are the longest, being about one-third of an inch 
in length, and they project -14 of an inch beneath the edge. 
At their bases there is a short subsidiary row of obliquely 
transverse lamellae. In these several respects they resemble 
the plates of baleen in the mouth of a whale. But towards 
the extremity of the beak they differ much, as they pro- 
ject inwards, instead of straight downwards. The entire 
head of the shoveller, though incomparably less bulky, is 
about one-eighteenth of the length of the head of a mod- 
erately large Balsenoptera rostrata, in which species the 
baleen is only nine inches long; so that if we were to make 


the head of the shoveller as long as that of the Balsenoptera, 
the lamellae would be six inches in length, — that is, two-thirds 
of the length of the baleen in this species of whale. The 
lower mandible of the shoveller-duck is furnished with 
lamellae of equal length with those above, but finer; and in 
being thus furnished it differs conspicuously from the lower 
jaw of a whale, which is destitute of baleen. On the other 
hand, the extremities of these lower lamellae are frayed into 
fine bristly points, so that they thus curiously resemble the 
plates of baleen. In the genus Prion, a member of the dis- 
tinct family of the Petrels, the upper mandible alone is fur- 
nished with lamellae, which are well developed and project 
beneath the margin; so that the beak of this bird resembles 
in this respect the mouth of a whak. 

From the highly developed structure of the shoveller's 
beak we may proceed (as I have learnt from information and 
specimens sent to me by Mr. Salvin), without any great 
break, as far as fitness for sifting is concerned, through the 
beak of the Merganetta armata, and in some respects through 
that of the Aix sponsa, to the beak of the common duck. 
In this latter species, the lamellae are much coarser than 
in the shoveller, and are firmly attached to the sides of the 
mandible ; they are only about 50 in number on each side, and 
do not project at all beneath the margin. They are square- 
topped, and are edged with translucent hardish tissue, as if 
for crushing food. The edges of the lower mandible are 
crossed by numerous fine ridges, which project very little. 
Although the beak is thus very inferior as a sifter to that 
of the shoveller, yet this bird, as every one knows, constantly 
uses it for this purpose. There are other species, as I hear 
from Mr. Salvin, in which the lamellae are considerably less 
developed than in the common duck; but I do not know 
whether they use their beaks for sifting the water. 

Turning to another group of the same family. In the 
Egyptian goose (Chenalopex) the beak closely resembles that 
of the common duck; but the lamellae are not so numerous, 
nor so distinct from each other, nor do they project so much 
inwards ; yet this goose, as I am informed by Mr. E. Bartlett, 
"uses its bill like a duck by throwing the waters out at the 
corners." Its chief food, however, is grass, which it crops 


like the common goose. In this latter bird, the lamellae of the 
upper mandible are much coarser than in the common duck, 
almost confluent, about 27 in number on each side, and ter- 
minating upwards in teeth-like knobs. The palate is also 
covered with hard rounded knobs. The edges of the lower 
mandible are serrated with teeth much more prominent, 
coarser, and sharper than in the duck. The common goose 
does not sift the water, but uses its beak exclusively for tear- 
ing or cutting herbage, for which purpose it is so well fitted, 
that it can crop grass closer than almost any other animal. 
There are other species of geese, as I hear from Mr. Bartlett, 
in which the lamellae are less developed than in the common 

We thus see that a member of the duck family, with a beak 
constructed like that of the common goose and adapted solely 
for grazing, or even a member with a beak having less well- 
developed lamellae, might be converted by small changes into 
a species like the Egyptian goose, — this into one like the com- 
mon duck, — and, lastly, into one like the shoveller, provided 
with a beak almost exclusively adapted for sifting the water; 
for this bird could hardly use any part of its beak, except 
the hooked tip, for seizing or tearing solid food. The beak 
of a goose, as I may add, might also be converted by small 
changes into one provided with prominent, recurved teeth, 
like those of the Merganser (a member of the same family), 
serving for the widely different purpose of securing live fish. 

Returning to the whales. The Hyperoodon bidens is desti- 
tute of true teeth in an efficient condition, but its palate is 
roughened, according to Lacepede, with small, unequal, hard 
points of horn. There is, therefore, nothing improbable in 
supposing that some early Cetacean form was provided with 
similar points of horn on the* palate, but rather more regu- 
larly placed, and which, like the knobs on the beak of the 
goose, aided it in seizing or tearing its food. If so, it will 
hardly be denied that the points might have been converted 
through variation and natural selection into lamellae as well- 
developed as those of the Egyptian goose, in which case they 
would have been used both for seizing objects and for sift- 
ing the water; then into lamellae like those of the domestic 
duck; and so onwards, until they became as well constructed 


as those of the shoveller, in which case they would have 
served exclusively as a sifting apparatus. From this stage, 
in which the lamellae would be two-thirds of the length of 
the plates of baleen in the Balaenoptera rostrata, gradations, 
which may be observed in still-existing Cetaceans, lead us 
onwards to the enormous plates of baleen in the Greenland 
whale. Nor is there the least reason to doubt that each step 
in this scale might have been as serviceable to certain an- 
cient Cetaceans, with the functions of the parts slowly chang- 
ing during the progress of development, as are the grada- 
tions ill the beaks of the different existing members of the 
duck family. We should bear in mind that each species of 
duck is subjected to a severe struggle for existence, and that 
the structure of every part of its frame must be well adapted 
to its conditions of life. 

The Pleuronectidse, or Flat-fish, are remarkable for their 
asymmetrical bodies. They rest on one side, — in the greater 
number of species on the left, but in some on the right side; 
and occasionally reversed adult specimens occur. The lower, 
or resting-surface, resembles at first sight the ventral sur- 
face of an ordinary fish : it is of a white color, less developed 
in many ways than the upper side, with the lateral fins often 
of smaller size. But the eyes offer the most remarkable pecu- 
liarity; for they are both placed on the upper side of the 
head. During early youth, however, they stand opposite to 
each other, and the whole body is then symmetrical, with 
both sides equally coloured. Soon the eye proper to the 
lower side begins to glide slowly round the head to the upper 
side; but does not pass right through the skull, as was for- 
merly thought to be the case. It is obvious that unless the 
lower eye did thus travel round, it could not be used by the 
fish whilst lying in its habitual position on one side. The 
lower eye would, also, have been liable to be abraded by the 
sandy bottom. That the Pleuronectidae are admirably adapted 
by their flattened and asymmetrical structure for their habits 
of life, is manifest from several species, such as soles, floun- 
ders, &c., being extremely common. The chief advantages 
thus gained seem to be protection from their enemies, and 
facility for feeding on the ground. The different members, 
however, of the family present, as Schiodte remarks, "a. long 


series of forms exhibiting a gradual transition from Hippo- 
glossus pinguis, which does not in any considerable degree 
alter the shape in which it leaves the ovum, to the soles, 
which are entirely thrown to one side." 

Mr. Mivart has taken up this case, and remarks that a 
sudden spontaneous transformation in the position of the 
eyes is hardly conceivable, in which I quite agree with him. 
He then adds: "if the transit was gradual, then how such 
transit of one eye a minute fraction of the journey towards 
the other side of the head could benefit the individual is, in- 
deed, far from clear. It seems, even, that such an incipient 
transformation must rather have been injurious." But he 
might have found an answer to this objection in the excel- 
lent observations published in 1867 by Malm. The Pleuro- 
nectidas, whilst very young and still symmetrical, with their 
eyes standing on opposite sides of the head, cannot long re- 
tain a vertical position, owing to the excessive depth of their 
bodies, the small size of their lateral fins, and to their being 
destitute of a swimbladder. Hence soon growing tired, they 
fall to the bottom on one side. Whilst thus at rest they often 
twist, as Malm observed, the lower eye upwards, to see above 
them; and they do this so vigorously that the eye is pressed 
hard against the upper part of the orbit. The forehead be- 
tween the eyes consequently becomes, as could be plainly 
seen, temporarily contracted in breadth. On one occasion 
Malm saw a young fish raise and depress the lower eye 
through an angular distance of about seventy degrees. 

We should remember that the skull at this early age is car- 
tilaginous and flexible, so that it readily yields to muscular 
action. It is also known with the higher animals, even after 
early youth, that the skull yields and is altered in shape, if 
the skin or muscles be permanently contracted through dis- 
ease or some accident. With long-eared rabbits, if one ear 
lops forwards and downwards, its weight drags forward all 
the bones of the skull on the same side, of which I have given 
a figure. Malm states that the newly hatched young of 
perches, salmon, and several other symmetrical fishes, have 
the habit of occasionally resting on one side at the bottom; 
and he has observed that they often then strain their lower 
eyes so as to look upwards; and their skulls arc thus rcn- 


dered rather crooked. These fishes, however, are soon able 
to hold themselves in a vertical position, and no permanent 
effect is thus produced. With the Pleuronectidse, on the 
other hand, the older they grow the more habitually they 
rest on one side, owing to the increasing flatness of their 
bodies, and a permanent effect is thus produced on the form 
of the head, and on the position of the eyes. Judging from 
analogy, the tendency to distortion would no doubt be in- 
creased through the principle of inheritance. Schiodte be- 
lieves, in opposition to some other naturalists, that the Pleu- 
ronectidae are not quite symmetrical even in the embryo; and 
if this be so, we could understand how it is that certain spe- 
cies, whilst young, habitually fall over and rest on the left 
side, and other species on the right side. Malm adds, in con- 
firmation of the above view, that the adult Trachypterus arc- 
ticus, which is not a member of the Pleuronectidse, rests on 
its left side at the bottom, and swims diagonally through the 
water; and in this fish, the two sides of the head are said to 
be somewhat dissimilar. Our great authority on Fishes, Dr. 
Giinther, concludes his abstract of Malm's paper, by remark- 
ing that "the author gives a very simple explanation of the 
abnormal condition of the Pleuronectoids." 

We thus see that the first stages of the transit of the eye 
from one side of the head to the other, which Mr. Mivart 
considers would be injurious, may be attributed to the habit, 
no doubt beneficial to the individual and to the species, of 
endeavouring to look upwards with both eyes, whilst resting 
on one side at the bottom. We may also attribute to the in- 
herited effects of use the fact of the mouth in several kinds 
of flat-fish being bent towards the lower surface, with the 
jaw bones stronger and more effective on this, the eyeless 
side of the head, than on the other, for the sake, as Dr. Tra- 
quair supposes, of feeding with ease on the ground. Disuse, 
on the other hand, will account for the less developed con- 
dition of the whole inferior half of the body, including the 
lateral fins; though Yarrell thinks that the reduced size of 
these fins is advantageous to the fish, as "there is so much 
less room for their action, than with the larger fins above." 
Perhaps the lesser number of teeth in the proportion of four 
to seven in the upper halves of the two jaws of the plaice, to 


twenty-five to thirty in the lower halves, may likewise be 
accounted for by disuse. From the colourless state of the 
ventral surface of most fishes and of many other animals, we 
may reasonably suppose that the absence of colour in flat- 
fish on the side, whether it be the right or left, which is 
undermost, is due to the exclusion of light. But it cannot 
be supposed that the peculiar speckled appearance of the 
upper side of the sole, so like the sandy bed of the sea, or 
the power in some species, as recently shown by Pouchet, of 
changing their colour in accordance with the surrounding 
surface, or the presence of bony tubercles on the upper side 
of the turbot, are due to the action of the light. Here natural 
selection has probably come into play, as well as in adapting 
the general shape of the body of these fishes, and many other 
peculiarities, to their habits of life. We should keep in mind, 
as I have before insisted, that the inherited effects of the 
increased use of parts, and perhaps of their disuse, will be 
strengthened by natural selection. For all spontaneous varia- 
tions in the right direction will thus be preserved; as will 
those individuals which inherit in the highest degree the 
effects of the increased and beneficial use of any part. How 
much to attribute in each particular case to the effects of use, 
and how much to natural selection, it seems impossible to 

I may give another instance of a structure which appar- 
endy owes its origin exclusively to use or habit. The ex- 
tremity of the tail in some American monkeys has been con- 
verted into a wonderfully perfect prehensile organ, and 
serves as a fifth hand. A reviewer who agrees with Mr. 
Mivart in every detail, remarks on this structure: "It is im- 
possible to believe that in any number of ages the first slight 
incipient tendency to grasp could preserve the lives of the 
individuals possessing it, or favour their chance of having 
and of rearing offspring." But there is no necessity for any 
such belief. Habit, and this almost implies that some benefit 
great or small is thus derived, would in all probability suffice 
for the work. Brehm saw the young of an African monkey 
(Cercopithecus) clinging to the under surface of their mother 
by their hands, and at the same time they hooked their little 
tails round that of their mother. Professor Henslow kept in 


confinement some harvest mice (Mus messorius) which do 
not possess a structurally prehensile tail; but he frequently 
observed that they curled their tails round the branches of a 
bush placed in the cage, and thus aided themselves in climb- 
ing. I have received an analogous account from Dr. Giin- 
ther, who has seen a mouse thus suspend itself. If the har- 
vest mouse had been more strictly arboreal, it would perhaps 
have had its tail rendered structurally prehensile, as is the 
case with some members of the same order. Why Cercopi- 
thecus, considering its habits whilst young, has not become 
thus provided, it would be difficult to say. It is, however, 
possible that the long tail of this monkey may be of more 
service to it as a balancing organ in making its prodigious 
leaps, than as a prehensile organ. 

The mammary glands are common to the whole class of 
mammals, and are indispensable for their existence ; they 
must, therefore, have been developed at an extremely remote 
period, and we can know nothing positively about their man- 
ner of development. Mr. Mivart asks : "Is it conceivable 
that the young of any animal was ever saved from destruction 
by accidentally sucking a drop of scarcely nutritious fluid 
from an accidentally hypertrophied cutaneous gland of its 
mother? And even if one was so, what chance was there of 
the perpetuation of such a variation?" But the case is not 
here put fairly. It is admitted by most evolutionists that 
mammals are descended from a marsupial form; and if so, 
the mammary glands will have been at first developed within 
the marsupial sack. In the case of the fish (Hippicampus) 
the eggs are hatched, and the young are reared for a time, 
within a sack of this nature ; and an American naturalist, 
Mr. Lockwood, believes from what he has seen of the devel- 
opment of the young, that they are nourished by a secretion 
from the cutaneous glands of the sack. Now with the early 
progenitors of mammals, almost before they deserved to be 
thus designated, is it not at least possible that the young 
might have been similarly nourished? And in this case, the 
individuals which secreted a fluid, in some degree or manner 
the most nutritious, so as to partake of the nature of milk, 
would in the long run have reared a larger number of well- 


nourished offspring, than would the individuals which se- 
creted a poorer fluid; and thus the cutaneous glands, which 
are the homologues of the mammary glands, would have been 
improved or rendered more effective. It accords with the 
widely extended principle of specialisation, that the glands 
over a certain space of the sack should have become more 
highly developed than the remainder ; and they would then 
have formed a breast, but at first without a nipple, as we see 
in the Ornithorhyncus, at the base of the mammalian series. 
Through what agency the glands over a certain space be- 
came more highly specialised than the others, I will not pre- 
tend to decide, whether in part through compensation of 
growth, the effects of use, or of natural selection. 

The development of the mammary glands would have been 
of no service, and could not have been effected through nat- 
ural selection, unless the young at the same time were able 
to partake of the secretion. There is no greater difficulty in 
understanding how young mammals have instinctively learnt 
to suck the breast, than in understanding how unhatched 
chickens have learnt to break the egg-shell by tapping against 
it with their specially adapted beaks ; or how a few hours 
after leaving the shell they have learnt to pick up grains of 
food. In such cases the most probable solution seems to be, 
that the habit was at first acquired by practice at a more ad- 
vanced age, and afterwards transmitted to the offspring at an 
earlier age. But the young kangaroo is said not to suck, 
only to cling to the nipple of its mother, who has the power 
of injecting milk into the mouth of her helpless, half-formed 
offspring. On this head Mr. Mivart remarks : "Did no spe- 
cial provision exist, the young one must infallibly be choked 
by the intrusion of the milk into the windpipe. But there is 
a special provision. The larynx is so elongated that it rises 
up into the posterior end of the nasal passage, and is thus 
enabled to give free entrance to the air for the lungs, while 
the milk passes harmlessly on each side of this elongated 
larynx, and so safely attains the gullet behind it." Mr. Mi- 
vart then asks how did natural selection remove in the adult 
kangaroo (and in most other mammals, on the assumption 
that they are descended from a marsupial form), "this at 
least perfectly innocent and harmless structure?" It may 


be suggested in answer that the voice, which is certainly of 
high importance to many animals, could hardly have been 
used with full force as long as the larynx entered the nasal 
passage ; and Professor Flower has suggested to me that this 
structure would have greatly interfered with an animal swal- 
lowing solid food. 

We will now turn for a short space to the lower divisions 
of the animal kingdom. The Echinodermata (star-fishes, 
sea-urchins, &c.) are furnished with remarkable organs, 
called pedicellarise, which consist, when well developed, of a 
tridactyle forceps— that is, of one formed of three serrated 
arms, neatly fitting together and placed on the summit of a 
flexible stem, moved by muscles. These forceps can seize 
firmly hold of any object; and Alexander Agassiz has seen 
an Echinus or sea-urchin rapidly passing particles of excre- 
ment from forceps to forceps down certain lines of its body, 
in order that its shell should not be fouled. But there is no 
doubt that besides removing dirt of all kinds, they subserve 
other functions; and one of these apparently is defence. 

With respect to these organs, Mr. Mivart, as on so many 
previous occasions, asks : "What v/culd be the utility of the 
iirst rtiSmentary beginnings of such structures, and how 
could such incipient buddings have ever preserved the life of 
a single Echinus ?" He adds, "not even the sudden develop- 
ment of the snapping action could have been beneficial with- 
out the freely moveable stalk, nor could the latter have been 
efficient without the snapping jaws, yet no minute merely in- 
definite variations could simultaneously evolve these complex 
co-ordinations of structure ; to deny this seems to do no less 
than to affirm a startling paradox." Paradoxical as this may 
appear to Mr. Mivart, tridactyle forcepses, immovably fixed 
at the base, but capable of a snapping action, certainly exist 
on some star-fishes; and this is intelligible if they serve, at 
least in part, as a means of defence. Mr. Agassiz, to whose 
great kindness I am indebted for much information on the 
subject, informs me that there are other star-fishes, in which 
one of the three arms of the forceps is reduced to a support 
for the other two; and again, other genera in which the third 
arm is completely lost. In Echinoneus, the shell is described 
by M. Perrier as bearing two kinds of pedicellarise, one re- 


sembling those of Echinus, and the other those of Spatan- 
gus ; and such cases arc always interesting as affording the 
means of apparently sudden transitions, through the abortion 
of one of the two states of an organ. 

With respect to the steps by which these curious organs 
have been evolved, Mr. Agassiz infers from his own re- 
searches and those of Miiller, that both in star-fishes and sea- 
urchins the pedicellariae must undoubtedly be looked at as 
modified spines. This may be inferred from their manner of 
development in the individual, as well as from a long and 
perfect series of gradations in different species and genera, 
from simple granules to ordinary spines, to perfect tridactyle 
pedicellaricT. The gradation extends even to the manner in 
which ordinary spines and the pedicellariae with their sup- 
porting calcareous rods are articulated to the shell. In cer- 
tain genera of star-fishes, "the very combinations needed to 
show that the pedicellariae are only modified branching spines" 
may be found. Thus we have fixed spines, with three equi- 
distant, serrated, moveable branches, articulated to near their 
bases; and higher up, on the same spine, three other move- 
able branches. Now when the latter arise from the summit 
of a spine they form in fact a rude tridactyle pedicellaria, 
and such may be seen on the same spine together with the 
three lower branches. In this case the identity in nature be- 
tween the arms of the pedicellariae and the moveable branches 
of a spine, is unmistakable. It is generally admitted that the or- 
dinary spines serve as a protection ; and if so, there can be 
no reason to doubt that those furnished with serrated and 
moveable branches likewise serve for the same purpose; and 
they would thus serve still more effectively as soon as by 
meeting together they acted as a prehensile or snapping ap- 
paratus. Thus every gradation, from an ordinary fixed spine 
to a fixed pedicellaria, would be of service. 

In certain genera of star-fishes these organs, instead of 
being fixed or borne on an immovable support, arc placed on 
the summit of a flexible and muscular, though short, stem; 
and in this case they probably subserve some additional func- 
tion besides defence. In the sea-urchins the steps can be fol- 
lowed by which a fixed spine becomes articulated to the shell, 
and is thus rendered moveable. I wish I had space here to 


give a fuller abstract of Mr. Agassiz's interesting observa- 
tions on the development of the pedicellarise. All possible 
gradations, as he adds, may likewise be found between the 
pedicellariae of the star-fishes and the hooks of the Ophiuri- 
ans, another group of the Echinodermata ; and again between 
the pedicellariae of sea-urchins and the anchors of the Holo- 
thurise, also belonging to the same great class. 

Certain compound animals, or zoophytes as they have been 
termed, namely the Polyzoa, are provided with curious or- 
gans called avicularia. These differ much in structure in the 
different species. In their most perfect condition, they curi- 
ously resemble the head and beak of a vulture in miniature, 
seated on a neck and capable of movement, as is likewise the 
lower jaw or mandible. In one species observed by me all the 
avicularia on the same branch often moved simultaneously 
backwards and forwards, with the lower jaw widely open, 
through an angle of about 90°, in the course of five seconds; 
and their movement caused the whole polyzoary to tremble. 
When the jaws are touched with a needle they seize it so 
firmly that the branch can thus be shaken. 

Mr. Mivart adduces this case, chiefly on account of the 
supposed difficulty of organs, namely the avicularia of the 
Polyzoa and the pedicellariae of the Echinodermata, which 
he considers as "essentially similar," having been developed 
through natural selection in widely distinct divisions of the 
animal kingdom. But, as far as structure is concerned, I can 
see no similarity between tridactyle pedicellariae and avicu- 
laria. The latter resemble somewhat more closely the chelae 
or pincers of Crustaceans; and Mr. Mivart might have ad- 
duced with equal appropriateness this resemblance as a special 
difficulty ; or even their resemblance to the head and beak of 
a bird. The avicularia are believed by Mr. Busk, Dr. Smitt, 
and Dr. Nitsche — naturalists who have carefully studied this 
group — to be homologous with the zooids and their cells 
which compose the zoophyte ; the moveable lip or lid of the 
cell corresponding with the lower and moveable mandible of 
the avicularium. Mr. Busk, however, does not know of any 
gradations now existing between a zooid and an avicularium. 
It is therefore impossible to conjecture by what serviceable 


gradations the one could have been converted into the other : 
but it by no means follows from this that such gradations 
have not existed. 

As the chela: of Crustaceans resemble in some degree the 
avicularia of Polyzoa, both serving as pincers, it may be 
worth while to show that with the former a long series of 
serviceable gradations still exists. In the first and simplest 
stage, the terminal segment of a limb shuts down either on 
the square summit of the broad penultimate segment, or 
against one whole side ; and is thus enabled to catch hold of 
an object; but the limb still serves as an organ of locomotion. 
We next find one corner of the broad penultimate segment 
slightly prominent, sometimes furnished with irregular teeth; 
and against these the terminal segment shuts down. By an 
increase in the size of this projection, with its shape, as well 
as that of the terminal segment, slightly modified and im- 
proved, the pincers are rendered more and more perfect, un- 
til we have at last an instrument as efificient as the chelse of 
a lobster; and all these gradations can be actually traced. 

Besides the avicularia, the Polyzoa possess curious organs 
called vibracula. These generally consist of long bristles, 
capable of movement and easily excited. In one species ex- 
amined by me the vibracula were slightly curved and ser- 
rated along the outer margin ; and all of them on the same 
polyzoary often moved simultaneously ; so that, acting like 
long oars, they swept a branch rapidly across the object- 
glass of my microscope. When a branch was placed on its 
face, the vibracula became entangled, and they made violent 
efforts to free themselves. They are supposed to serve as a 
defence, and may be seen, as Mr. Busk remarks, "to sweep 
slowly and carefully over the surface of the polyzoary, re- 
moving what might be noxious to the delicate inhabitants of 
the cells when their tentacula are protruded." The avicu- 
laria, like the vibracula, probably serve for defence, but they 
?lso catch and kill small living animals, which it is believed 
are afterwards swept by the currents within reach of the 
tentacula of the zooids. Some species are provided with 
avicularia and vibracula ; some with avicularia alone, and a 
few with vibracula alone. 

It is not easy to imagine two objects more widely different 


in appearance than a bristle or vibraculum, and an avicu- 
larium like the head of a bird ; yet they are almost certainly 
homologous and have been developed from the same common 
source, namely a zooid with its cell. Hence we can under- 
stand how it is that these organs graduate in some cases, as 
I am informed by Mr. Busk, into each other. Thus with the 
avicularia of several species of Lepralia, the moveable 
mandible is so much produced and is so like a bristle, 
that the presence of the upper or fixed beak alone serves 
to determine its avicularian nature. The vibracula may 
have been directly developed from the lips of the cells, 
without having passed through the avicularian stage; but 
it seems more probable that they have passed through this 
stage, as during the early stages of the transformation, the 
other parts of the cell with the included zooid could hardly 
have disappeared at once. In many cases the vibracula have 
a grooved support at the base, which seems to represent the 
fixed beak; though this support in some species is quite ab- 
sent. This view of the development of the vibracula, if trust- 
worthy, is interesting; for supposing that all the species pro- 
vided with avicularia had become extinct, no one with the 
most vivid imagination would ever have thought that the 
vibracula had originally existed as part of an organ, resem- 
bling a bird's head or an irregular box or hood. It is inter- 
esting to see two such widely different organs developed from 
a common origin; and as the moveable lip of the cell serves 
as a protection to the zooid, there is no difficulty in believing 
that all the gradations, by which the lip became converted 
first into the lower mandible of an avicularium and then into 
an elongated bristle, likewise served as a protection in differ- 
ent ways and under different circumstances. 

In the vegetable kingdom Mr. Mivart only alludes to two 
cases, namely the structure of the flowers of orchids, and the 
movements of climbing plants. With respect to the former, 
he says, "the explanation of their origin is deemed thoroughly 
unsatisfactory— utterly insufficient to explain the incipient, 
infinitesimal beginnings of structures which are of utility 
only when they are considerably developed." As I have 
fully treated this subject in another work, I will here give 


only a few details on one alone of the most striking pecu- 
liarities of the flowers of orchids, namely their pollinia. A 
pollinium when highly developed consists of a mass of pollen- 
grains, affixed to an elastic foot-stalk or caudicle, and this 
to a little mass of extremely viscid matter. The pollinia are 
by this means transported by insects from one flower to the 
stigma of another. In some orchids there is no caudicle to 
the pollen-masses, and the grains are merely tied together by 
fine threads ; but as these are not confined to orchids, they 
need not here be considered; yet I may mention that at the 
base of the orchidaceous series, in Cypripedium, we can see 
how the threads were probably first developed. In other 
orchids the threads cohere at one end of the pollen-masses ; 
and this forms the first or nascent trace of a caudicle. That 
this is the origin of the caudicle, even when of considerable 
length and highly developed, we have good evidence in the 
aborted pollen-grains which can sometimes be detected 
embedded within the central and solid parts. 

With respect to the second chief peculiarity, namely the 
little mass of viscid matter attached to the end of the caudicle, 
a long series of gradations can be specified, each of plain 
service to the plant. In most flowers belonging to other 
orders the stigma secretes a little viscid matter. Now in cer- 
tain orchids similar viscid matter is secreted, but in much 
larger quantities by one alone of the three stigmas ; and this 
stigma, perhaps in consequence of the copious secretion, is 
rendered sterile. When an insect visits a flower of this kind, 
it rubs off some of the viscid matter and thus at the same 
time drags away some of the pollen-grains. From this 
simple condition, which differs but little from that of a mul- 
titude of common flowers, there are endless gradations, — to 
species in which the pollen-mass terminates in a very short, 
free caudicle, — to others in which the caudicle becomes firmly 
attached to the viscid matter, with the sterile stigma itself 
much modified. In this latter case we have a pollinium in its 
most highly developed and perfect condition. He who will 
carefully examine the flowers of orchids for himself will not 
deny the existence of the above series of gradations — from a 
mass of pollen-grains merely tied together by threads, with 
the stigma differing but little from that of an ordinary flower. 


to a highly complex pollinium, admirably adapted for trans- 
portal by insects ; nor will he deny that all the gradations in 
the several species are admirably adapted in relation to the 
general structure of each flower for its fertilisation by differ- 
ent insects. In this, and in almost every other case, the en- 
quiry may be pushed further backwards ; and it may be asked 
how did the stigma of an ordinary flower become viscid, but 
as we do not know the full history of any one group of be- 
ings, it is as useless to ask, as it is hopeless to attempt 
answering, such questions. 

We will now turn to climbing plants. These can be ar- 
ranged in a long series, from those which simply twine round 
a support, to those which I have called leaf-climbers, and to 
those provided with tendrils. In these two latter classes the 
stems have generally, but not always, lost the power of twin- 
ing, though they retain the power of revolving, which the 
tendrils likewise possess. The gradations from leaf-climbers 
to tendril-bearers are wonderfully close, and certain plants 
may be indifferently placed in either class. But in ascending 
the series from simple twiners to leaf-climbers, an important 
quality is added, namely sensitiveness to a touch, by which 
means the foot-stalks of the leaves or flowers, or these modi- 
fied and converted into tendrils, are excited to bend round 
and clasp the touching object. He who will read my memoir 
on these plants will, I think, admit that all the many grada- 
tions in function and structure between simple twiners and 
tendril-bearers are in each case beneficial in a high degree to 
the species. For instance, it is clearly a great advantage to 
a twining plant to become a leaf-climber; and it is probable 
that every twiner which possessed leaves with long foot- 
stalks would have been developed into a leaf-climber, if the 
foot-stalks had possessed in any slight degree the requisite 
sensitiveness to a touch. 

As twining is the simplest means of ascending a support, 
and forms the basis of our series, it may naturally be asked 
how did plants acquire this power in an incipient degree, 
afterwards to be improved and increased through natural se- 
lection. The power of twining depends, firstly, on the stems 
whilst young being extremely flexible (but this is a character 
common to many plants which are not climbers) ; and, sec- 


ondly, on their continually bending to all points of the com- 
pass, one after the other in succession, in the same order. By 
this movement the stems are inclined to all sides, and are 
made to move round and round. As soon as the lower part 
of a stem strikes against any object and is stopped, the upper 
part still goes on bending and revolving, and thus necessarily 
twines round and up the support. The revolving movement 
ceases after the early growth of each shoot. As in many 
widely separated families of plants, single species and single 
genera possess the power of revolving, and have thus become 
twiners, they must have independently acquired it, and cannot 
have inherited it from a common progenitor. Hence I was 
led to predict that some slight tendency to a movement of this 
kind would be found to be far from uncommon with plants 
which did not climb ; and that this had afforded the basis for 
natural selection to work on and improve. When I made 
this prediction, I knew of only one imperfect case, namely of 
the young flower-peduncles of a Maurandia v/hich revolved 
slightly and irregularly, like the stems of twining plants, but 
without making any use of this habit. Soon afterwards 
Fritz Miiller discovered that the young stems of an Alisma 
and of a Linum, — plants which do not climb and are widely 
separated in the natural system, — revolved plainly, though 
irregularly ; and he states that he has reason to suspect that 
this occurs with some other plants. These slight movements 
appear to be of no service to the plants in question ; anyhow, 
they are not of the least use in the way of climbing, which 
is the point that concerns us. Nevertheless we can see that 
if the stems of these plants had been flexible, and if under the 
conditions to which they are exposed it had profited them to 
ascend to a height, then the habit of slightly and irregularly 
revolving might have been increased and utilised through 
natural selection, until they had become converted into well- 
developed twining species. 

With respect to the sensitiveness of the foot-stalks of the 
leaves and flowers, and of tendrils, nearly the same remarks 
are applicable as in the case of the revolving movements of 
twining plants. As a vast number of species, belonging to 
widely distinct groups, are endowed with this kind of sensi- 
tiveness, it ought to be found in a nascent condition in many 


plants which have not become climbers. This is the case: I 
observed that the young flower-peduncles of the above Mau- 
randia curved themselves a little towards the side which was 
touched. Morren found in several species of Oxalis that the 
leaves and their foot-stalks moved, especially after exposure 
to a hot sun, when they were gently and repeatedly touched, 
or when the plant was shaken. I repeated these observations 
on some other species of Oxalis with the same result; in 
some of them the movement was distinct, but was best seen 
in the young leaves ; in others it was extremely slight. It is 
a more important fact that according to the high authority 
of Hofmeister, the young shoots and leaves of all plants move 
after being shaken; and with climbing plants it is, as we 
know, only during the early stages of growth that the foot- 
stalks and tendrils are sensitive. 

It is scarcely possible that the above slight movements, due 
to a touch or shake, in the young and growing organs of 
plants, can be of any functional importance to them. But 
plants possess, in obedience to various stimuli, powers of 
movement, which are of manifest importance to them; for 
instance, towards and more rarely from the light, — in oppo- 
sition to, and more rarely in the direction of, the attraction 
of gravity. When the nerves and muscles of an animal are 
excited by galvanism or by the absorption of strychnine, the 
consequent movements may be called an incidental result, for 
the nerves and muscles have not been rendered specially sen- 
sitive to these stimuli. So with plants it appears that, from 
having the power of movement in obedience to certain stim- 
uli, they are excited in an incidental manner by a touch, or 
by being shaken. Hence there is no great difficulty in ad- 
mitting that in the case of leaf-climbers and tendril-bearers, 
it is this tendency which has been taken advantage of and in- 
creased through natural selection. It is, however, probable, 
from reasons which I have assigned in my memoir, that this 
will have occurred only with plants which had already ac- 
quired the power of revolving, and had thus become twiners. 

I have already endeavoured to explain how plants became 
twiners, namely, by the increase of a tendency to slight and 
irregular revolving movements, which were at first of no use 
to them; this movement, as well as that due to a touch or 


shake, being the incidental result of the power of moving, 
gained for other and beneficial purposes. Whether, during 
the gradual development of climbing plants, natural selection 
has been aided by the inherited efifects of use, I will not pre- 
tend to decide ; but we know that certain periodical move- 
ments, for instance the so-called sleep of plants, are governed 
by habit. 

I have now considered enough, perhaps more than enough, 
of the cases, selected with care by a skilful naturalist, to 
prove that natural selection is incompetent to account for the 
incipient stages of useful structures ; and I have shown, as I 
hope, that there is no great difficulty on this head. A good 
opportunity has thus been afforded for enlarging a little on 
gradations of structure, often associated with changed func- 
tions, — an important subject, which was not treated at suf- 
ficient length in the former editions of this work. I will now 
briefly recapitulate the foregoing cases. 

With the giraffe, the continued preservation of the indi- 
viduals of some extinct high-reaching ruminant, which had 
the longest necks, legs, &c., and could browse a little above 
the average height, and the continued destruction of those 
which could not browse so high, would have sufficed for the 
production of this remarkable quadruped; but the prolonged 
use of all the parts together with inheritance will have aided 
in an important manner in their co-ordination. With the 
many insects which imitate various objects, there is no im- 
probability in the belief that an accidental resemblance to 
some common object was in each case the foundation for the 
work of natural selection, since perfected through the occa- 
sional preservation of slight variations which made the re- 
semblance at all closer; and this will have been carried on 
as long as the insect confinued to vary, and as long as a more 
and more perfect resemblance led to its escape from sharp- 
sighted enemies. In certain species of whales there is a ten- 
dency to the formation of irregular little points of horn on 
the palate; and it seems to be quite within the scope of nat- 
ural selection to preserve all favourable variations, until the 
points were converted first into lamellated knobs or teeth, 
like those on the beak of a goose, — then into short lamellae. 


like those of the domestic di'cks — and then into lamellae, as 
perfect as those of the shoveller-duck, — and finally into the 
gigantic plates of baleen, as in the mouth of the Greenland 
whale. In the family of the ducks, the lamellse are first used 
as teeth, then partly as teeth and partly as a sifting ap- 
paratus, and at last almost exclusively for this latter purpose. 

With such structures as the above lamellae of horn or whale- 
bone, habit or use can have done little or nothing, as far as 
we can judge, towards their development. On the other 
hand, the transportal of the lower eye of a flat-fish to the 
upper side of the head, and the formation of a prehensile tail, 
may be attributed almost wholly to continued use, together 
with inheritance. With respect to the mammae of the higher 
animals, the most probable conjecture is that primordially 
the cutaneous glands over the whole surface of a marsupial 
sack secreted a nutritious fluid ; and that these glands were 
improved in function through natural selection, and concen- 
trated into a confined area, in which case they would have 
formed a mamma. There is no more difficulty in under- 
standing how the branched spines of some ancient Echino- 
derm, which served as a defence, became developed through 
natural selection into tridactyle pedicellariae, than in under- 
standing the development of the pincers of crustaceans, 
through slight, serviceable modifications in the ultimate and 
penultimate segments of a limb, which was at first used solely 
for locomotion. In the avicularia and vibracula of the 
Polyzoa we have organs widely different in appearance de- 
veloped from the same source ; and with the virbracula we 
can understand how the successive gradations might have 
been of service. With the pollinia of orchids, the threads 
which originally served to tie together the pollen-grains, can 
be traced cohering into caudicles; and the steps can likewise 
be followed by which viscid matter, such as that secreted by 
the stigmas of ordinary flowers, and still subserving nearly 
but not quite the same purpose, became attached to the free 
ends of the caudicles; — all these gradations being of mani- 
fest benefit to the plants in question. With respect to climb- 
ing plants, I need not repeat what has been so lately said. 

It has often been asked, if natural selection be so potent, 
why has not this or that structure been gained by certain 


species, to which it would apparently have been advantage- 
ous? But it is unreasonable to expect a precise answer to 
such questions, considering our ignorance of the past history 
of each species, and of the conditions which at the present 
day determine its numbers and range. In most cases only 
general reasons, but in some few cases special reasons, can 
be assigned. Thus to adapt a species to new habits of life, 
many co-ordinated modifications are almost indispensable, 
and it may often have happened that the requisite parts did 
not vary in the right manner or to the right degree. Many 
species must have been prevented from increasing in numbers 
through destructive agencies, which stood in no relation to 
certain structures, which we imagine would have been gained 
through natural selection from appearing to us advantageous 
to the species. In this case, as the struggle for life did not 
depend on such structures, they could not have been acquired 
through natural selection. In many cases complex and long- 
enduring conditions, often of a peculiar nature, are necessary 
for the development of a structure ; and the requisite con- 
ditions may seldom have concurred. The belief that any 
given structure, which we think, often erroneously, would 
have been beneficial to a species, would have been gained 
under all circumstances through natural selection, is opposed 
to what we can understand of its manner of action. Mr. 
Mivart does not deny that natural selection has effected 
something; but he considers it as "demonstrably insufficient" 
to account for the phenomena which I explain by its agency. 
His chief arguments have now been considered, and the 
others will hereafter be considered. They seem to me to par- 
take little of the character of demonstration, and to have 
little weight in comparison with those in favour of the power 
of natural selection, aided by the other agencies often speci- 
fied. I am bound to add, that some of the facts and argu- 
ments here used by me, have been advanced for the same 
purpose in an able article lately published in the 'Mcdico- 
Chirurgical Review.' 

At the present day almost all naturalists admit evolution 
under some form. Mr. Mivart believes that species change 
through "an internal force or tendency," about which it is 
not pretended that anything is known. That species have a 



capacity for change will be admitted by all evolutionists ; but 
there is no need, as it seems to me, to invoke any internal 
force beyond the tendency to ordinary variability, which 
through the aid of selection by man has given rise to many 
well-adapted domestic races, and which through the aid of 
natural selection would equally well give rise by graduated 
steps to natural races or species. The final result will gen- 
erally have been, as already explained, an advance, but in 
some few cases a retrogression, in organisation. 

Mr. Mivart is further inclined to believe, and some nat- 
uralists agree with him, that new species manifest themselves 
"with suddenness and by modifications appearing at once." 
For instance, he supposes that the differences between the 
extinct three-toed Hipparion and the horse arose suddenly. 
He thinks it difficult to believe that the wing of a bird "was 
developed in any other way than by a comparatively sudden 
modification of a marked and important kind;" and appa- 
rently he would extend the same view to the wings of bats 
and pterodactyles. This conclusion, which implies great 
breaks or discontinuity in the series, appears to me improb- 
able in the highest degree. 

Every one who believes in slow and gradual evolution, will 
of course admit that specific changes may have been as abrupt 
and as great as any single variation which we meet with 
under nature, or even under domestication. But as species 
are more variable when domesticated or cultivated than under 
their natural conditions, it is not probable that such 
great and abrupt variations have often occurred under 
nature, as are known occasionally to arise under domestica- 
tion. Of these latter variations several may be attributed to 
reversion; and the characters which thus reappear were, it 
is probable, in many cases at first gained in a gradual man- 
ner. A still greater number must be called monstrosities, 
such as six-fingered men, porcupine men, Ancon sheep, Niata 
cattle, &c. ; and as they are widely different in character from 
natural species, they throw very little light on our subject. 
Excluding such cases of abrupt variations, the few which re- 
main would at best constitute, if found in a state of nature, 
doubtful species, closely related to their parental types. 

My reasons for doubting whether natural species have 


changed as abruptly as have occasionally domestic races, and 
for entirely disbelieving that they have changed in the won- 
derful manner indicated by Mr. Mivart, are as follows. Ac- 
cording to our experience, abrupt and strongly marked vari- 
ations occur in our domesticated productions, singly and at 
rather long intervals of time. If such occurred under na- 
ture, they would be liable, as formerly explained, to be lost 
by accidental causes of destruction and by subsequent inter- 
crossing; and so it is known to be under domestication, un- 
less abrupt variations of this kind are specially preserved and 
separated by the care of man. Hence in order that a new 
species should suddenly appear in the manner supposed by 
Mr. Mivart, it is almost necessary to believe, in opposition to 
all analogy, that several wonderfully changed individuals 
appeared simultaneously within the same district. This dif- 
ficulty, as in the case of unconscious selection by man, is 
avoided on the theory of gradual evolution, through the pres- 
ervation of a large number of individuals, which varied more 
or less in any favourable direction, and of the destruction of 
a large number which varied in an opposite manner. 

That many species have been evolved in an extremely 
gradual manner, there can hardly be a doubt. The species 
and even the genera of many large natural families are so 
closely allied together, that it is difficult to distinguish not a 
few of them. On every continent in proceeding from north 
to south, from lowland to upland, &c., we meet with a host 
of closely related or representative species ; as we likewise 
do on certain distinct continents, which we have reason to 
believe were formerly connected. But in making these and 
the following remarks, I am compelled to allude to subjects 
hereafter to be discussed. Look at the many outlying islands 
round a continent, and see how many of their inhabitants can 
be raised only to the rank of doubtful species. So it is if we 
look to past times, and compare the species which have just 
passed away with those still living within the same areas; or 
if we compare the fossil species embedded in the sub-stages 
of the same geological formation. It is indeed manifest that 
multitudes of species are related in the closest manner to 
other species that still exist, or have lately existed ; and it will 
hardly be maintained that such species have been developed 


in an abrupt or sudden manner. Nor should it be forgotten, 
when we look to the special parts of allied species, instead of 
to distinct species, that numerous and wonderfully fine grada- 
tions can be traced, connecting together widely different 

Many large groups of facts are intelligible only on the 
principle that species have been evolved by very small steps. 
For instance, the fact that the species included in the larger 
genera are more closely related to each other, and present a 
greater number of varieties than do the species in the smaller 
genera. The former are also grouped in little clusters, like 
varieties round species ; and they present other analogies with 
varieties, as was shown in our second chapter. On this same 
principle we can understand how it is that specific characters 
are more variable than generic characters ; and how the parts 
which are developed in an extraordinary degree or manner 
are more variable than other parts of the same species. 
Many analogous facts, all pointing in the same direction, 
could be added. 

Although very many species have almost certainly been 
produced by steps not greater than those separating fine vari- 
eties ; yet it may be maintained that some have been devel- 
oped in a different and abrupt manner. Such an admission, 
however, ought not to be made without strong evidence being 
assigned. The vague and in some respects false analogies, 
as they have been shown to be by Mr. Chauncey Wright, 
which have been advanced in favour of this view, such as the 
sudden crystallisation of inorganic substances, or the falling 
of a facetted spheroid from one facet to another, hardly de- 
serve consideration. One class of facts, however, namely, the 
sudden appearance of new and distinct forms of life in our geo- 
logical formations supports at first sight the belief in abrupt 
development. But the value of this evidence depends entirely 
on the perfection of the geological record, in relation to 
periods remote in the history of the world. If the record is 
as fragmentary as many geologists strenuously assert, there 
is nothing strange in new forms appearing as if suddenly 

Unless we admit transformations as prodigious as those 
advocated by Mr. Mivart, such as the sudden development of 


the wings of birds or bats, or the sudden conversion of a 
Hipparion into a horse, hardly any Hght is thrown by the be- 
lief in abrupt modifications on the deficiency of connecting 
links in our geological formations. But against the belief in 
such abrupt changes, embryology enters a strong protest. It 
is notorious that the wings of birds and bats, and the legs of 
horses or other quadrupeds, arc undistinguishable at an early 
embryonic period, and that they become differentiated by in- 
sensibly fine steps. Embryological resemblances of all kinds 
can be accounted for, as we shall hereafter see, by the pro- 
genitors of our existing species having varied after early 
youth, and having transmitted their newly acquired char- 
acters to their offspring, at a corresponding age. The em- 
bryo is thus left almost unaffected, and serves as a record of 
the past condition of the species. Hence it is that existing 
species during the early stages of their development so often 
resemble ancient and extinct forms belonging to the same 
class. On this view of the meaning of embryological resem- 
blances, and indeed on any view, it is incredible that an ani- 
mal should have undergone such momentous and abrupt trans- 
formations, as those above indicated ; and yet should not bear 
even a trace in its embryonic condition of any sudden modi- 
fication ; every detail in its structure being developed by in- 
sensibly fine steps. 

He who believes that some ancient form was transformed 
suddenly through an internal force or tendency into, for in- 
stance, one furnished with wings, will be almost compelled 
to assume, in opposition to all analogy, that many individuals 
varied simultaneously. It cannot be denied that such abrupt 
and great changes of structure are widely different from 
those which most species apparently have undergone. He 
will further be compelled to believe that many structures 
beautifully adapted to all the other parts of the same creature 
and to the surrounding conditions, have been suddenly pro- 
duced ; and of such complex and wonderful co-adaptations, 
he will not be able to assign a shadow of an explanation. 
He will be forced to admit that these great and sudden trans- 
formations have left no trace of their action on the embryo. 
To admit all this is, as it seems to me, to enter into the 
realms of miracle, and to leave those of Science. 


Instincts comparable with habits, but different in their origin — In- 
stincts graduated — Aphides and ants — Instincts variable — Do- 
mestic instincts, their origin — Natural instincts of the cuckoo, 
molothrus, ostrich, and parasitic bees — Slave-making ants — Hive- 
bee, its cell-making instinct — Changes of instinct and structure 
not necessarily simultaneous — DifBculties of the theory of the 
Natural Selection of instincts — Neuter or sterile insects — 

MANY instincts are so wonderful that their develop- 
ment will probably appear to the reader a diflficulty 
sufficient to overthrow my whole theory. I may 
here premise, that I have nothing to do with the origin of 
the mental powers, any more than I have ^vith that of life 
itself. We are concerned only with the diversities of instinct 
and of the other mental faculties in animals of the same class. 

I will not attempt any definition of instinct. It would be 
easy to show that several distinct mental actions are com- 
monly embraced by this term; but every one understands 
what is meant, when it is said that instinct impels the cuckoo 
to migrate and to lay her eggs in other birds' nests. An ac- 
tion, which we ourselves require experience to enable us to 
perform, when performed by an animal, more especially by a 
very young one, without experience, and when performed by 
many individuals in the same way, without their knowing 
for what purpose it is performed, is usually said to be in- 
stinctive. But I could show that none of these characters 
are universal. A little dose of judgment or reason, as Pierre 
Huber expresses it, often comes into play, even with animals 
low in the scale of nature. 

Frederick Cuvier and several of the older metaphysicians 
have compared instinct with habit. This comparison gives, 
I think, an accurate notion of the frame of m-ind under 



which an instinctive action is performed, but not necessarily 
of its origin. How unconsciously many habitual actions are 
performed, indeed not rarely in direct opposition to our con- 
scious will ! yet they may be modified by the will or reason. 
Habits easily become associated with other habits, with cer- 
tain periods of time, and states of the body. When once 
acquired, they often remain constant throughout life. Sev- 
eral other points of resemblance between instincts and habits 
could be pointed out. As in repeating a well-known song, so 
in instincts, one action follows another by a sort of rhythm; 
if a person be interrupted in a song, or in repeating anything 
by rote, he is generally forced to go back to recover the 
habitual train of thought; so P. Huber found it was with a 
caterpillar, which makes a very complicated hammock; for if 
he took a caterpillar which had completed its hammock up to, 
say, the sixth stage of construction, and put it into a ham- 
mock completed up only to the third stage, the caterpillar 
simply re-performed the fourth, fifth, and sixth stages of 
construction. If, however, a caterpillar were taken out of a 
hammock made up, for instance, to the third stage, and were 
put into one finished up to the sixth stage, so that much of 
its work was already done for it, far from deriving any bene- 
fit from this, it was much embarrassed, and in order to com- 
plete its hammock, seemed forced to start from the third 
stage, where it had left off, and thus tried to complete the 
already finished work. 

If we suppose any habitual action to become inherited — and 
it can be shown that this does sometimes happen — then the 
resemblance between what originally was a habit and an in- 
stinct becomes so close as not to be distinguished. If Mozart, 
instead of playing the pianoforte at three years old with won- 
derfully litde practice, had played a tune with no practice at 
all, he might truly be said to have done so instinctively. But 
it would be a serious error to suppose that the greater num- 
ber of instincts have been acquired by habit in one genera- 
tion, and then transmitted by inheritance to succeeding gen- 
erations. It can be clearly shown that the most wonderful 
instincts with which we are acquainted, namely, those of the 
hive-bee and of many ants, could not oossibly have been ac- 
quired by habit. 


It will be universally admitted that instincts are as im- 
portant as corporeal structures for the welfare of each spe- 
cies, under its present conditions of life. Under changed con- 
ditions of life, it is at least possible that sHght modifications 
of instinct might be profitable to a species; and if it can be 
shown that instincts do vary ever so little, then I can see no 
difficulty in natural selection preserving and continually accu- 
mulating variations of instinct to any extent that was profit- 
able. It is thus, as I believe, that all the most complex and 
wonderful instincts have originated. As modifications of 
corporeal structure arise from, and are increased by, use or 
habit, and are diminished or lost by disuse, so I do not doubt 
it has been with instincts. But I believe that the effects of 
habit are in many cases of subordinate importance to the 
effects of the natural selection of what may be called spon- 
taneous variations of instincts; — that is of variations pro- 
duced by the same unknown causes which produce slight 
deviations of bodily structure. 

No complex instinct can possibly be produced through 
natural selection, except by the slow and gradual accumula- 
tion of numerous slight, yet profitable, variations. Hence, as 
in the cases of corporeal structures, we ought to find in 
nature, not the actual transitional gradations by which each 
complex instinct has been acquired — for these could be found 
only in the lineal ancestors of each species — but we ought to 
find in the collateral lines of descent some evidence of such 
gradations ; or we ought at least be able to show that grada- 
tions of some kind are possible ; and this we certainly can do. 
I have been surprised to find, making allowance for the in- 
stincts of animals having been but little observed except in 
Europe and North America, and for no instinct being known 
amongst extinct species, how very generally gradations, lead- 
ing to the most complex instincts, can be discovered. Changes 
of instinct may sometimes be facilitated by the same species 
having different instincts at different periods of life, or at 
different seasons of the year, or when placed under different 
circumstances, &c. ; in which case either the one or the other 
instinct might be preserved by natural selection. And such 
instances of diversity of instinct in the same species can be 
shown to occur in nature. 


Again, as in the case of corporeal structure, and conform- 
ably to my theory, the instinct of each species is good for 
itself, but has never, as far as we can judge, been produced 
for the exclusive good of others. One of the strongest in- 
stances of an animal apparently performing an action for the 
sole good of another, with which I am acquainted, is that of 
aphides voluntarily yielding, as was first observed by Huber, 
their sweet excretion to ants : that they do so voluntarily, the 
following facts show. I removed all the ants from a group 
of about a dozen aphides on a dock-plant, and prevented their 
attendance during several hours. After this interval, I felt 
sure that the aphides would want to excrete. I watched them 
for some time through a lens, but not one excreted; I then 
tickled and stroked them with a hair in the same manner, as 
well as T could, as the ants do with their antennae ; but not one 
excreted. Afterwards I allowed an ant to visit them, and it 
immediately seemed, by its eager way of running about, to be 
well aware what a rich flock it had discovered ; it then began 
to play with its antennae on the abdomen first of one aphis and 
then of another ; and each, as soon as it felt the antennae, 
immediately lifted up its abdomen and excreted a limpid drop 
of sweet juice, which was eagerly devoured by the ant. Even 
the quite young aphides behaved in this manner, showing that 
the action was instinctive, and not the result of experience. 
It is certain, from the observations of Huber, that the aphides 
show no dislike to the ants : if the latter be not present they 
are at last compelled to eject their excretion. But as the ex- 
cretion is extremely viscid, it is no doubt a convenience to 
the aphides to have it removed ; therefore probably they do 
not excrete solely for the good of the ants. Although there 
is no evidence that any animal performs an action for the 
exclusive good of another species, yet each tries to take ad- 
vantage of the instincts of others, as each takes advantage 
of the weaker bodily structure of other species. So again 
certain instincts cannot be considered as absolutely perfect; 
but as details on this and other such points are not indis- 
pensable, they may be here passed over. 

As some degree of variation in instincts under a state of 
nature, and the inheritance of such variations, are indis- 
pensable for the action of natural selection, as many instances 


as possible ought to be given ; but want of space prevents me. 
I can only assert that instincts certainly do vary — for in- 
stance, the migratory instinct, both in extent and direction, 
and in its total loss. So it is with the nests of birds, which 
vary partly in dependence on the situations chosen, and on 
the nature and temperature of the country inhabited, but 
often from causes wholly unknown to us : Audubon has given 
several remarkable cases of differences in the nests of the 
same species in the northern and southern United States. 
Why, it has been asked, if instinct be variable, has it not 
granted to the bee "the ability to use some other material 
when wax was deficient"? But what other natural material 
could bees use? They will work, as I have seen, with wax 
hardened with vermilion or softened with lard. Andrew 
Knight observed that his bees, instead of laboriously collect- 
ing propolis, used a cement of wax and turpentine, with 
which he had covered decorticated trees. It has lately been 
shown that bees, instead cf searching for pollen, will gladly 
use a very different substance, namely oatmeal. Fear of any 
particular enemy is certainly an instinctive quality, as may 
be seen in nestling birds, though it is strengthened by experi- 
ence, and by the sight of fear of the same enemy in other 
animals. The fear of man is slowly acquired, as I have else- 
where shown, by the various animals which inhabit desert 
islands; and we see an instance of this even in England, in 
the greater wildness of all our large birds in comparison with 
our small birds ; for the large birds have been most persecuted 
by man. We may safely attribute the greater wildness of our 
large birds to this cause ; for in uninhabited islands large birds 
are not more fearful than small ; and the magpie, so wary in 
England, is tame in Norway, as is the hooded crow in Egypt. 
That the mental qualities of animals of the same kind, born 
in a state of nature, vary much, could be shown by many 
facts. wSeveral cases could also be adduced of occasional and 
strange habits in wild animals, which, if advantageous to the 
species, might have given rise, through natural selection, to 
new instincts. But I am well aware that these general state- 
ments, without the facts in detail, will produce but a feeble 
effect on the reader's mind. I can only repeat my assurance, 
that I do not speak without good evidence. 



The possibility, or even probability, of inherited variations 
of instinct in a state of nature will be strengthened by briefly 
considering a few cases under domestication. We shall thus 
be enabled to see the part which habit and the selection of so- 
called spontaneous variations have played in modifying the 
mental qualities of our domestic animals. It is notorious 
how much domestic animals vary in their mental qualities. 
With cats, for instance, one naturally takes to catching rats, 
and another, and these tendencies are known to be in- 
herited. One cat, according to Mr. St. John, always brought 
home game-birds, another hares or rabbits, and another 
hunted on marshy ground and almost nightly caught wood- 
cocks or snipes. A number of curious and authentic instances 
could be given of various shades of disposition and of taste, 
and likewise of the oddest tricks, associated with certain 
frames of mind or periods of time, being inherited. But let 
us look to the familiar case of the breeds of the dogs: it can- 
not be doubted that young pointers (I have myself seen a 
striking instance) will sometimes point and even back other 
dogs the very first time that they are taken out; retrieving 
is certainly in some degree inherited by retrievers; and a ten- 
dency to run round, instead of at, a flock of sheep, by shep- 
herd dogs. I cannot see that these actions, performed without 
experience by the young, and in nearly the same manner by 
each individual, performed with eager delight by each breed, 
and without the end being known — for the young pointer can 
no more know that he points to aid his master, than the white 
butterfly knows why she lays her eggs on the leaf of the cab- 
bage — I cannot see that these actions differ essentially from 
true instincts. If we were to behold one kind of wolf, when 
young and without any training, as soon as it scented its prey, 
stand motionless like a statue, and then slowly crawl forward 
with a peculiar gait; and another kind of wolf rushing round, 
instead of at, a herd of deer, and driving them to a distant 
point, we should assuredly call these actions instinctive. 
Domestic instincts, as they may be called, are certainly far 
less fixed than natural instincts; but they have been acted on 


by far less rigorous selection, and have been transmitted for 
an incomparably shorter period, under less fixed conditions 
of life. 

How strongly these domestic instincts, habits, and disposi- 
tions are inherited, and how curiously they become mingled, 
is well shown when different breeds of dogs are crossed. 
Thus it is known that a cross with a bull-dog has affected for 
many generations the courage and obstinacy of greyhounds; 
and a cross with a greyhound has given to a whole family of 
shepherd-dogs a tendency to hunt hares. These domestic in- 
stincts, when thus tested by crossing, resemble natural in- 
stincts, which in a like manner become curiously blended 
together, and for a long period exhibit traces of the instincts 
of either parent: for example, Le Roy describes a dog, whose 
great-grandfather was a wolf, and this dog showed a trace 
of its wild parentage only in one way, by not coming in a 
straight line to his master, when called. 

Domestic instincts are sometimes spoken of as actions which 
have become inherited solely from long-continued and com- 
pulsory habit; but this is not true. No one would ever have 
thought of teaching, or probably could have taught, the 
tumbler-pigeon to tumble, — an action which, as I have wit- 
nessed, is performed by young birds, that have never seen a 
pigeon tumble. We may believe that some one pigeon showed 
a slight tendency to this strange habit, and that the long- 
continued selection of the best individuals in successive gen- 
erations made tumblers what they now are ; and near Glasgow 
there are house-tumblers, as I hear from Mr. Brent, which 
cannot fly eighteen inches high without going head over 
heels. It may be doubted whether any one would have 
thought of training a dog to point, had not some one dog 
naturally shown a tendency in this line; and this is known 
occasionally to happen, as I once saw, in a pure terrier: the 
act of pointing is probably, as many have thought, only the 
exaggerated pause of an animal preparing to spring on its 
prey. When the first tendency to point was once displayed, 
methodical selection and the inherited effects of compulsory 
training in each successive generation would soon complete 
the work; and unconscious selection is still in progress, as 
each man tries to procure, without intending to improve the 


breed; dogs which stand and hunt best. On the other hand, 
habit alone in some cases has sufficed; hardly any animal is 
more difficult to tame than the young of the wild rabbit; 
scarcely any animal is tamer than the young of the tame rab- 
bit; but I can hardly suppose that domestic rabbits have often 
been selected for tamencss alone ; so that we must attribute at 
least the greater part of the inherited change from extreme 
wildness to extreme tameness, to habit and long-continued 
close confinement. 

Natural instincts are lost under domestication : a remark- 
able instance of this is seen in those breeds of fowls which 
very rarely or never become "broody," that is, never wish to 
sit on their eggs. Familiarity alone prevents our seeing how 
largely and how permanently the minds of our domestic ani- 
mals have been modified. It is scarcely possible to doubt that 
the love of man has become instinctive in the dog. All wolves, 
foxes, jackals, and species of the cat genus, when kept tame, 
are most eager to attack poultry, sheep, and pigs; and this 
tendency has been found incurable in dogs which have been 
brought home as puppies from countries such as Tierra del 
Fuego and Australia, where the savages do not keep these 
domestic animals. How rarely, on the other hand, do our 
civilised dogs, even when quite young, require to be taught 
not to attack poultry, sheep, and pigs ! No doubt they occa- 
sionally do make an attack, and are then beaten; and if not 
cured, they are destroyed ; so that habit and some degree of 
selection have probably concurred in civilising by inheritance 
our dogs. On the other hand, young chickens have lost, 
wholly by habit, that fear of the dog and cat which no doubt 
was originally instinctive in them; for I am informed by 
Captain Hutton that the young chickens of the parent-stock, 
the Callus bankiva, when reared in India under a hen, are at 
first excessively wild. So it is with young pheasants reared 
in England under a hen. It is not that chickens have lost all 
fear, but fear only of dogs and cats, for if the hen gives the 
danger-chuckle, they will run (more especially young tur- 
keys) from under her, and conceal themselves in the sur- 
rounding grass or thickets; and this is evidently done for the 
instinctive purpose of allowing, as we see in wild ground- 
birds, their mother to fly away. But this instinct retained by 


our chickens has become useless under domestication, for the 
mother-hen has almost lost by disuse the power of flight. 

Hence, we may conclude, that under domestication instincts 
have been acquired, and natural instincts have been lost, 
partly by habit, and partly by man selecting and accumulating, 
during successive generations, peculiar mental habits and ac- 
tions, which at first appeared from what we must in our 
ignorance call an accident. In some cases compulsory habit 
alone has sufficed to produce inherited mental changes; in 
other cases, compulsory habit has done nothing, and all has 
been the result of selection, pursued both methodically and 
unconsciously: but in most cases habit and selection have 
probably concurred. 


We shall, perhaps, best understand how instincts in a state 
of nature have become modified by selection, by considering 
a few cases. I will select only three, — namely, the instinct 
which leads the cuckoo to lay her eggs in other birds' nests; 
the slave-making instinct of certain ants ; and the cell-making 
power of the hive-bee. These two latter instincts have gener- 
ally and justly been ranked by naturalists as the most won- 
derful of all known instincts. 

Instincts of the Cuckoo. — It is supposed by some naturalists 
that the more immediate cause of the instinct of the cuckoo 
is, that she lays her eggs, not daily, but at intervals of two 
or three days; so that, if she were to make her own nest and 
sit on her own eggs, those first laid would have to be left for 
some time unincubated, or there would be eggs and young 
birds of different ages in the same nest. If this were the 
case, the process of laying and hatching might be inconveni- 
ently long, more especially as she migrates at a very early 
period; and the first hatched young would probably have to 
be fed by the male alone. But the American cuckoo is in this 
predicament; for she makes her own nest, and has eggs and 
young successively hatched, all at the same time. 'It has been 
both asserted and denied that the American cuckoo occasion- 
ally lays her eggs in other birds' nests ; but I have lately heard 
from Dr. Merrell, of Iowa, that he once found in Illinois a 
young cuckoo together with a young jay in the nest of a Blue 


jay (Garrulus cristatus) ; and as both were nearly full feath- 
ered, there could be no mistake in their identification. I could 
also give several instances of various birds which have been 
known occasionally to lay their eggs in other birds' nests. 
Now let us suppose that the ancient progenitor of our Euro- 
pean cuckoo had the habits of the American cuckoo, and that 
she occasionally laid an egg in another bird's nest. If the 
old bird profited by this occasional habit through being enabled 
to migrate earlier or through any other cause; or if the young 
were made more vigorous by advantage being taken of the 
mistaken instinct of another species than when reared by their 
own mother, encumbered as she could hardly fail to be by 
having eggs and young of different ages at the same time J 
then the old birds or the fostered young would gain an ad- 
vantage. And analogy would lead us to believe, that the 
young thus reared would be apt to follow by inheritance the 
occasional and aberrant habit of their mother, and in their 
turn would be apt to lay their eggs in other birds' nests, and 
thus be more successful in rearing their young. By a con- 
tinued process of this nature, I believe that the strange in- 
stinct of our cuckoo has been generated. It has, also, re- 
cently been ascertained on sufficient evidence, by Adolf 
Miiller, that the cuckoo occasionally lays her eggs on the bare 
ground, sits on them, and feeds her young. This rare event is 
probably a case of reversion to the long-lost, aboriginal in- 
stinct of nidification. 

It has been objected that I have not noticed other related 
instincts and adaptations of structure in the cuckoo, which 
are spoken of as necessarily co-ordinated. But in all cases, 
speculation on an instinct known to us only in a single species, 
is useless, for we have hitherto had no facts to guide us. 
Until recently the instincts of the European and of the non- 
parasitic American cuckoo alone were known; now, owing to 
Mr. Ramsay's observations, we have learnt something about 
three Australian species, which lay their eggs in other birds' 
nests. The chief points to be referred to are three: first, that 
the common cuckoo, with rare exceptions, lays only one egg 
in a nest, so that the large and voracious young bird receives 
ample food. Secondly, that the eggs are remarkably small, 
not exceeding those of the skylark, — a bird about one-fourth 


as large as the cuckoo. That the small size of the egg is a 
real case of adaptation we may infer from the fact of 
the non-parasitic American cuckoo laying full-sized eggs. 
Thirdly, that the young cuckoo, soon after birth, has the in- 
stinct, the strength, and a properly shaped back for ejecting 
its foster-brothers, which then perish from cold and hunger. 
This has been boldly called a beneficial arrangement, in order 
that the young cuckoo may get sufficient food, and that its 
foster-brothers may perish before they had acquired much 
feeling ! 

Turning now to the Australian species; though these birds 
generally lay only one egg in a nest, it is not rare to find two 
and even three eggs in the same nest. In the Bronze cuckoo 
the eggs vary greatly in size, from eight to ten lines in length. 
Now if it had been of an advantage to this species to have 
laid eggs even smaller than those now laid, so as to have de- 
ceived certain foster-parents, or, as is more probable, to have 
been hatched within a shorter period (for it is asserted that 
there is a relation between the size of eggs and the period of 
their incubation), then there is no difficulty in believing that 
a race or species might have been formed which would have 
laid smaller and smaller eggs; for these would have been 
more safely hatched and reared. Mr. Ramsay remarks that 
two of the Australian cuckoos, when they lay their eggs in 
an open nest, manifest a decided preference for nests con- 
taining eggs similar in colour to their own. The European 
species apparently manifests some tendency towards a similar 
instinct, but not rarely departs from it, as is shown by her 
laying her dull and pale-coloured eggs in the nest of the 
Hedge-warbler with bright greenish-blue eggs. Had our 
cuckoo invariably displayed the above instinct, it would as- 
suredly have been added to those which it is assumed must 
all have been acquired together. The eggs of the Australian 
Bronze cuckoo vary, according to Mr. Ramsay, to an ex- 
traordinary degree in colour; so that in this respect, as well 
as in size, natural selection might have secured and fixed any 
advantageous variation. 

In the case of the European cuckoo, the offspring, of the 
foster-parents are commonly ejected from the nest within 
three days after the cuckoo is hatched; and as the latter at 


this age is in a most helpless condition, Mr. Gould was for- 
merly inclined to believe that the act of ejection was per- 
formed by the foster-parents themselves. But he has now re- 
ceived a trustworthy account of a young cuckoo which was 
actually seen, whilst still blind and not able even to hold up 
its own head, in the act of ejecting its foster-brothers. One 
of these was replaced in the nest by the observer, and was 
again thrown out. With respect to the means by which this 
strange and odious instinct was acquired, if it were of great 
importance for the young cuckoo, as is probably the case, to 
receive as much food as possible soon after birth, I can see 
no special difficulty in its having gradually acquired, during 
successive generations, the blind desire, the strength, and 
structure necessary for the work of ejection; for those young 
cuckoos which had such habits and structure best developed 
would be the most securely reared. The first step towards 
the acquisition of the proper instinct might have been mere 
unintentional restlessness on the part of the young bird, when 
somewhat advanced in age and strength ; the habit having 
been afterwards improved, and transmitted to an earlier age. 
I can see no more difficulty in this, than in the unhatched 
young of other birds acquiring the instinct to break through 
their own shells ; — or than in young snakes acquiring in their 
upper jaws, as Owen has remarked, a transitory sharp tooth 
for cutting through the tough egg-shell. For if each part is 
liable to individual variations at all ages, and the variations 
tend to be inherited at a corresponding or earlier age, — propo- 
sitions which cannot be disputed, — then the instincts and 
structure of the young could be slowly modified as surely as 
those of the adult; and both cases must stand or fall together 
with the whole theory of natural selection. 

Some species of Molothrus, a widely distinct genus of 
American birds, allied to our starlings, have parasitic habits 
like those of the cuckoo; and the species present an interest- 
ing gradation in the perfection of their instincts. The sexes 
of Molothrus badius are stated by an excellent observer, Mr. 
Hudson, sometimes to live promiscuously together in flocks, 
and sometimes to pair. They either build a nest of their own, 
or seize on one belonging to some other bird, occasionally 
throwing out the nestlings of the stranger. They either lay 


their eggs in the nest thus appropriated, or oddly enough build 
one for themselves on the top of it. They usually sit on their 
own eggs and rear their own young; but Mr. Hudson says it 
is probable that they are occasionally parasitic, for he has 
seen the young of this species following old birds of a distinct 
kind and clamouring to be fed by them. The parasitic habits 
of another species of Molothrus, the M. bonariensis, are much 
more highly developed than those of the last, but are still far 
from perfect. This bird, as far as it is known, invariably 
lays its eggs in the nests of strangers; but it is remarkable 
that several together sometimes commence to build an irregu- 
lar untidy nest of their own, placed in singularly ill-adapted 
situations, as on the leaves of a large thistle. They never, 
however, as far as Mr. Hudson has ascertained, complete a 
nest for themselves. They often lay so many eggs — from 
fifteen to twenty — in the same foster-nest, that few or none 
can possibly be hatched. They have, moreover, the extraordi- 
nary habit of pecking holes in the eggs, whether of their own 
species or of their foster-parents, which they find in the ap- 
propriated nests. They drop also many eggs on the bare 
ground, which are thus wasted. A third species, the M. pecoris 
of North America, has acquired instincts as perfect as those 
of the cuckoo, for it never lays more than one egg in a foster- 
nest, so that the young bird is securely reared. Mr. Hudson is 
a strong disbeliever in evolution, but he appears to have been 
so much struck by the imperfect instincts of the Molothrus 
bonariensis that he quotes my words, and asks, "Must we con- 
sider these habits, not as especially endowed or created in- 
stincts, but as small consequences of one general law, namely, 

"Various birds, as has already been remarked, occasionally 
lay their eggs in the nests of other birds. This habit is not 
very uncommon with the Gallinaceae, and throws some light 
on the singular instinct of the ostrich. In this family several 
hen-birds unite and lay first a few eggs in one nest and then 
in another ; and these are hatched by the males. This instinct 
may probably be accounted for by the fact of the hens laying 
a large number of eggs, but, as with the cuckoo, at intervals 
of two or three days. The instinct, however, of the American 
ostrich, as in the case of the Molothrus bonariensis, has not 



as yet been perfected; for a surprising number of eggs lie 
strewed over the plains, so that in one day's hunting I picked 
up no less than twenty lost and wasted eggs. 

Many bees are parasitic, and regularly lay their eggs in the 
nests of other kinds of bees. This case is more remarkable 
than that of the cuckoo; for these bees have not only had 
their instincts but their structure modified in accordance with 
their parasitic habits ; for they do not possess the pollen- 
collecting apparatus which would have been indispensable if 
they had stored up food for their own young. Some species 
of Sphegidae (wasp-like insects) are likewise parasitic; and 
M. Fabre has lately shown good reason for believing that, 
although the Tachytes nigra generally makes its own burrow 
and stores it with paralysed prey for its own larvae, yet that, 
when this insect finds a burrow already made and stored by 
another sphex, it takes advanta;^e of the prize, and becomes 
for the occasion parasitic. In this case, as with that of the 
Molothrus or cuckoo, I can see no difficulty in natural selec- 
tion making an occasional habit permanent, if of advantage 
to the species, and if the insect whose nest and stored food 
are feloniously appropriated, be not thus exterminated. 

Slave-making instinct. — This remarkable instinct was first 
discovered in the Formica (Polyerges) rufescens by Pierre 
Huber, a better observer even than his celebrated father. This 
ant is absolutely dependent on its slaves; without their aid, 
the species would certainly become extinct in a single year. 
The males and fertile females do no work of any kind, and 
the workers or sterile females, though most energetic and 
courageous in capturing slaves, do no other work. They are 
incapable of making their own nests, or of feeding their own 
larvae. When the old nest is found inconvenient, and they 
have to migrate, it is the slaves which determine the migra- 
tion, and actually carry their masters in their jaws. So 
utterly helpless are the masters, that when Huber shut up 
thirty of them without a slave, but with plenty of the food 
Vv'hich they like best, and with their own larvae and pupre to 
stimulate them to work, they did nothing; they could not 
even feed themselves, and many perished of hunger. Huber 
then introduced a single slave (F. fusca), and she instantly 
set to work, fed and saved the survivors; made some cells 


and tended the larvae, and put all to rights. What can be 
more extraordinary than these well-ascertained facts? If 
we had not known of any other slave-making ant, it would 
have been hopeless to speculate how so wonderful an instinct 
could have been perfected. 

Another species, Formica sanguinea, was likewise first dis- 
covered by P. Huber to be a slave-making ant. This species 
is found in the southern parts of England, and its habits 
have been attended to by Mr. F. Smith, of the British Mu- 
seum, to whom I am much indebted for information on this 
and other subjects. Although fully trusting to the statements 
of Huber and Mr. Smith, I tried to approach the subject in a 
sceptical frame of mind, as any one may well be excused for 
doubting the existence of so extraordinary an instinct as 
that of making slaves. Hence, I will give the observations 
which I made in some little detail. I opened fourteen nests 
of F. sanguinea, and found a few slaves in all. Males and 
fertile females of the slave species (F. fusca) are found 
only in their own proper communities, and have never been 
observed in the nests of F. sanguinea. The slaves are black 
and not above half the size of their red masters, so that the 
contrast in their appearance is great. When the nest is 
slightly disturbed, the slaves occasionally come out, and like 
their masters are much agitated and defend the nest: when 
the nest is much disturbed, and the larvse and pupae are ex- 
posed, the slaves work energetically together with their mas- 
ters in carrying them away to a place of safety. Hence, it 
is clear, that the slaves feel quite at home. During the 
months of June and July, on three successive years, I watched 
for many hours several nests in Surrey and Sussex, and 
never saw a slave either leave or enter a nest. As, during 
these months, the slaves are very few in number, I thought 
that they might behave differently when more numerous; but 
Mr. Smith informs me that he has watched the nests at 
various hours during May, June, and August, both in Surrey 
and Hampshire, and has never seen the slaves, though pres- 
ent in large numbers in August, either leave or enter the 
nest. Hence he considers them as strictly household slaves. 
The masters, on the other hand, may be constantly seen 
bringing in materials for the nest, and food of all kinds. 


During the year i860, however, in the month of July, I came 
across a community with an unusually large stock of slaves, 
and I observed a few slaves mingled with their masters 
leaving the nest, and marching along the same road to a tall 
Scotch-fir tree, twenty-five yards distant, which they ascended 
together, probably in search of aphides or cocci. According 
to Huber, who had ample opportunities for observation, the 
slaves in Switzerland habitually work with their masters in 
making the nest, and they alone open and close the doors in 
the morning and evening ; and, as Huber expressly states, 
their principal ofiice is to search for aphides. This differ- 
ence in the usual habits of the masters and slaves in the two 
countries, probably depends merely on the slaves being cap- 
tured in greater numbers in Switzerland than in England. 

One day I fortunately witnessed a migration of F. san- 
guinea from one nest to another, and it was a most interest- 
ing spectacle to behold the masters carefully carrying their 
slaves in their jaws instead of being carried by them, as in 
the case of F. rufescens. Another day my attention was 
struck by about a score of the slave-makers haunting the 
same spot, and evidently not in search of food; they ap- 
proached and were vigorously repulsed by an independent 
community of the slave-species (F. fusca) ; sometimes as 
many as three of these ants clinging to the legs of the slave- 
making F. sanguinea. The latter ruthlessly killed their small 
opponents, and carried their dead bodies as food to their 
nest, twenty-nine yards distant ; but they were prevented 
from getting any pupae to rear as slaves. I then dug up a 
small parcel of the pupae of F. fusca from another nest, and 
put them down on a bare spot near the place of combat ; 
they were eagerly seized and carried off by the tyrants, who 
perhaps fancied that, after all, they had been victorious in 
their late combat. 

At the same time I laid on the same place a small parcel 
of the pupx of another species, F. flava, with a few of these 
little yellow ants still clinging to the fragments of their 
nest. This species is sometimes, though rarely, made into 
slaves, as has been described by Mr. Smith. Althougli so 
small a species, it is very courageous, and I have seen it 
ferociously attack other ants. In one instance I found to my 


surprise an independent community of F. flava under a stone 
beneath a nest of the slave-making F. sanguinea; and when 
I had accidentally disturbed both nests, the little ants at- 
tacked their big neighbours with surprising courage. Now 
I was curious to ascertain whether F. sanguinea could dis-. 
tinguish the pupae of F. fusca, which they habitually make 
into slaves, from those of the little and furious F. flava, 
which they rarely capture, and it was evident that they did 
at once distinguish them; for we have seen that they eagerly 
and instantly seized the pupae of F. fusca, whereas they were 
much terrified when they came across the pupae, or even the 
earth from the nest, of F. flava, and quickly ran away; but 
in about a quarter of an hour, shortly after all the little yel- 
low ants had crawled away, they took heai/ and carried off 
the pupae. 

One evening I visited another community of F. sanguinea, 
and found a number of these ants returning home and enter- 
ing their nests, carrying the dead bodies of F. fusca (show- 
ing that it was not a migration) and numerous pupae. I 
traced a long file of ants burthened with booty, for about 
forty yards back, to a very thick clump of heath, whence I 
saw the last individual of F. sanguinea emerge, carrying a 
pupa ; but I was not able to find the desolated nest in the 
thick heath. The nest, however, must have been close at 
hand, for two or three individuals of F. fusca were rushing 
about in the greatest agitation, and one was perched motion- 
less with its own pupa in its mouth on the top of a spray 
of heath, an image of despair over its ravaged home. 

Such are the facts, though they did not need confirmation 
by me, in regard to the wonderful instinct of making slaves. 
Let it be observed what a contrast the instinctive habits of 
F. sanguinea present with those of the continental F. rufes- 
cens. The latter does not build its own nest, does not deter- 
mine its own migrations, does not collect food for itself or 
its young, and cannot even feed itself: it is absolutely depen- 
dent on its numerous slaves. Formica sanguinea, on the 
other hand, possesses much fewer slaves, and in the early 
part of the summer extremely few: the masters determine 
when and where a new nest shall be formed, and when they 
migrate, the masters carry the slaves. Both in Switzerland 


and England the slaves seem to have the exclusive care of 
the larvae, and the masters alone go on slave-making expe- 
ditions. In Switzerland the slaves and masters work to- 
gether, making and bringing materials for the nest; both, 
but chiefly the slaves, tend, and milk, as it may be called, 
their aphides; and thus both collect food for the community. 
In England the masters alone usually leave the nest to col- 
lect building materials and food for themselves, their slaves 
and larvas. So that the masters in this country receive much 
less service from their slaves than they do in Switzerland. 

By what steps the instinct of F. sanguinea originated I 
will not pretend to conjecture. But as ants which are not 
slave-makers will, as I have seen, carry off the pupa; of 
other species, if scattered near their nests, it is possible that 
such pupa; originally stored as food might become developed; 
and the foreign ants thus unintentionally reared would then 
follow their proper instincts, and do what work they could. 
If their presence proved useful to the species which had 
seized them — if it were more advantageous to this species 
to capture workers than to procreate them — the habit of col- 
lecting pupae, originally for food, might by natural selection 
be strengthened and rendered permanent for the very dif- 
ferent purpose of raising slaves. When the instinct was 
once acquired, if carried out to a much less extent even 
than in our British F. sanguinea, which, as we have seen, is 
less aided by its slaves than the same species in Switzerland, 
natural selection might increase and modify the instinct — 
always supposing each modification to be of use to the spe- 
cies-^until an ant was formed as abjectly dependent on its 
slaves as is the Formica rufescens. 

Cell-making instinct of the Hive-Bee. — I will not here 
enter on minute details on this subject, but will merely give 
an outline of the conclusions at which I have arrived. He 
must be a dull man who can examine the exquisite structure 
of a comb, so beautifully adapted to its end, without enthusi- 
astic admiration. We hear from mathematicians that bees 
have practically solved a recondite problem, and have made 
their cells of the proper shape to hold the greatest possible 
amount of honey, with the least possible consumption of 
precious wax in their construction. It has been remarked 


that a skilful workman with fitting tools and measures, 
would find it very difficult to make cells of wax of the true 
form, though this is effected by a crowd of bees working 
in a dark hive. Granting whatever instincts you please, it 
seems at first quite inconceivable how they can make all 
the necessary angles and planes, or even perceive when they 
are correctly made. But the difficulty is not nearly so great 
as it at first appears: all this beautiful work can be shown, 
I think, to follow from a few simple instincts. 

I was led to investigate this subject by Mr. Waterhouse, 
who has shown that the form of the ceil stands in close 
relation to the presence of adjoining cells; and the follow- 
ing view may, perhaps, be considered only as a modification 
of his theory. Let us look to the great principle of grada- 
tion, and see whether Nature does not reveal to us her 
method of work. At one end of a short series we have 
humble-bees, which use their old cocoons to hold honey, 
sometimes adding to them short tubes of wax, and likewise 
making separate and very irregular rounded cells of wax. 
At the other end of the series we have the cells of the hive- 
bee, placed in a double layer: each cell, as is well known, 
is an hexagonal prism, with the basal edges of its six sides 
bevelled so as to join an inverted pyramid, of three rhombs. 
These rhombs have certain angles, and the three which form 
the pyramidal base of a single cell on one side of the comb 
enter into the composition of the bases of three adjoining 
cells on the opposite side. In the series between the extreme 
perfection of the cells of the hive-bee and the simplicity of 
those of the humble-bee we have the cells of the Mexican 
Melipona domestica, carefully described and figured by Pierre 
Hwber. The Melipona itself is intermediate in structure be- 
tween the hive and humble-bee, but more nearly related to 
the latter ; it forms a nearly regular waxen comb of cylin- 
drical cells, in which the young are hatched, and, in addi- 
tion, some large cells of wax for holding honey. These 
latter cells are nearly spherical and of nearly equal sizes, and 
are aggregated into an irregular mass. But the important 
point to notice is, that these cells are always made at that 
degree of nearness to each other that they would have inter- 
sected or broken into each other if the spheres had been 


completed; but this is never permitted, the bees building per- 
fectly flat walls of wax between the spheres which thus 
tend to intersect. Hence, each cell consists of an outer 
spherical portion, and of two, three, or more flat surfaces, 
according as the cell adjoins two, three, or more other cells. 
When one cell rests on three other cells, which, from the 
spheres being nearly of the same size, is very frequently 
and necessarily the case, the three flat surfaces are united 
into a pyramid; and this pyramid, as Huber has remarked, 
is manifestly a gross imitation of the three-sided pyramidal 
base of the cell of the hive-bee. As in the cells of the hive- 
bee, so here, the three plane surfaces in any one cell neces- 
sarily enter into the construction of three adjoining cells. 
It is obvious that the Melipona saves wax, and what is more 
important, labour, by this manner of building; for the flat 
walls between the adjoining cells are not double, but are 
of the same thickness as the outer spherical portions, and 
yet each flat portion forms a part of two cells. 

Reflecting on this case, it occurred to me that if the Meli- 
pona had made its spheres at some given distance from each 
other, and had made them of equal sizes and had arranged 
them symmetrically in a double layer, the resulting structure 
would have been as perfect as the comb of the hive-bee. Ac- 
cordingly I wrote to Professor Miller of Cambridge, and 
this geometer has kindly read over the following statement, 
drawn up from his information, and tells me that it is 
strictly correct : — 

If a number of equal spheres be described with their 
centres placed in two parallel layers ; with the centre of each 
sphere at the distance of radius X y 2, or radius X 1.41421 
(or at some lesser distance), from the centres of the six 
surrounding spheres in the same layer; and at the same dis- 
tance from the centres of the adjoining spheres in the other 
and parallel layer; then, if planes of intersection between 
the several spheres in both layers be formed, there will re- 
sult a double layer of hexagonal prisms united together by 
pyramidal bases formed of three rhombs; and the rhombs 
and the sides of the hexagonal prisms will have every angle 
identically the same with the best measurements which have 
been made of the cells of the hive-bee. But I hear from 


Prof. Wyman, who has made numerous careful measure- 
ments, that the accuracy of the workmanship of the bee has 
been greatly exaggerated; so much so, that whatever the 
typical form of the cell may be, it is rarely, if ever, realised. 
Hence we may safely conclude that, if we could slightly 
modify the instincts already possessed by the Melipona, and 
in themselves not very wonderful, this bee would make a 
structure as wonderfully perfect as that of the hive-bee. We 
must suppose the Melipona to have the power of forming 
her cells truly spherical, and of equal sizes; and this would 
not be very surprising, seeing that she already does so to a 
certain extent, and seeing what perfectly cylindrical bur- 
rows many insects make in wood, apparently by turning 
round on a fixed point. We must suppose the Melipona to 
arrange her cells in level layers, as she already does her 
cylindrical cells; and we must further suppose, and this is 
the greatest difficulty, that she can somehow judge accu- 
rately at what distance to stand from her fellow-labourers 
when several are making their spheres; but she is already 
so far enabled to judge of distance, that she always describes 
her spheres so as to intersect to a certain extent; and then 
she unites the points of intersection by perfectly flat sur- 
faces. By such modifications of instincts which in them- 
selves are not very wonderful — hardly more wonderful than 
those which guide a bird to make its nest, — I believe that 
the hive-bee has acquired, through natural selection, her 
inimitable architectural powers. 

But this theory can be tested by experiment. Following 
the example of Mr. Tegetmeier, I separated two combs, 
and put between them a long, thick, rectangular strip of 
wax: the bees instantly began to excavate minute circular 
pits in it; and as they deepened these little pits, they made 
* them wider and wider until they were converted into shal- 
low basins, appearing to the eye perfectly true or parts of 
a sphere, and of about the diameter of a cell. It was most 
interesting to observe that, wherever several bees had be- 
gun to excavate these basins near together, they had begun 
their work at such a distance from each other, that by the 
time the basins had acquired the above-stated width (i. e. 
about the width of an ordinary cell), and were in depth 


about one-sixth of the diameter of the sphere of which they 
formed a part, the rims of the basins intersected or broke 
into each other. As soon as this occurred, the bees ceased to 
excavate, and began to build up flat walls of wax on the 
lines of intersection between the basins, so that each hex- 
agonal prism was built upon the scalloped edge of a smooth 
basin, instead of on the straight edges of a three-sided pyra- 
mid as in the case of ordinary cells. 

I then put into the hive, instead of a thick, rectangular 
piece of wax, a thin and narrow, knife-edged ridge, coloured 
with vermilion. The bees instantly began on both sides to 
excavate little basins near to each other, in the same way as 
before; but the ridge of wax was so thin, that the bottoms 
of the basins, if they had been excavated to the same depth 
as in the former experiment, would have broken into each 
other from the opposite sides. The bees, however, did not 
suffer this to happen, and they stopped their excavations in 
due time ; so that the basins, as soon as they had been a little 
deepened, came to have flat bases; and these flat bases, 
formed by thin little plates of the vermilion wax left un- 
gnawed, were situated, as far as the eye could judge, exactly 
along the planes of imaginary intersection between the 
basins on the opposite sides of the ridge of wax. In some 
parts, only small portions, in other parts, large portions of a 
rhombic plate were thus left between the opposed basins, 
but the work, from the unnatural state of things, had not 
been neatly performed. The bees must have worked at very 
nearly the same rate in circularly gnawing away and deep- 
ening the basins on both sides of the ridge of vermilion wax, 
in order to have thus succeeded in leaving flat plates between 
the basins, by stopping work at the planes of intersection. 

Considering how flexible thin wax is, I do not see that 
there is any difficulty in the bees, whilst at work on the two 
sides of a strip of wax, perceiving when they have gnawed 
the wax away to the proper thinness, and then stopping their 
work. In ordinary combs it has appeared to me that the 
bees do not always succeed in working at exactly the same 
rate from the opposite sides; for I have noticed half-com- 
pleted rhombs at the base of a just commenced cell, which 
•were slightly concave on one side, where I suppose that the 


bees had excavated too quickly, and convex on the opposed 
side where the bees had worked less quickly. In one well 
marked instance, I put the comb back into the hive, and 
allowed the bees to go on working for a short time, and 
again examined the cell, and I found that the rhombic plate 
had been completed, and had become perfectly flat: it was 
absolutely impossible, from the extreme thinness of the little 
plate, that they could have effected this by gnawing away 
the convex side ; and I suspect that the bees in such cases 
stand on opposite sides and push and bend the ductile and 
warm wax (which as I have tried is easily doiic) into its 
proper intermediate plane, and thus flatten it. 

From the experiment of the ridge of vermilion wax we 
can see that, if the bees were to build for themselves a thin 
wall of wax, they could make their cells of the proper shape, 
by standing at the proper distance from each other, by exca- 
vating at the same rate, and by endeavouring to make equal 
spherical hollows, but never allowing the spheres to break 
into each other. Now bees, as may be clearly seen by exam- 
ining the edge of a growing comb, do make a rough, circum- 
ferential wall or rim all round the comb ; and they gnaw this 
away from the opposite sides, always working circularly as 
they deepen each cell. They do not make the whole three- 
sided pyramidal base of any one cell at the same time, but 
only that one rhombic plate which stands on the extreme 
growing margin, or the two plates, as the case may be; and 
they never complete the upper edges of the rhombic plates, 
until the hexagonal walls are commenced. Some of these 
statements differ from those made by the justly celebrated 
elder Huber, but I am convinced of their accuracy; and if 
I had space, I could show that they are conformable with 
my theory. 

Ruber's statement, that the very first cell is excavated out 
of a little parallel-sided wall of wax, is not, as far as I have 
seen, strictly correct; the first commencement having always 
been a little hood of wax; but I will not here enter on de- 
tails. We see how important a part excavation plays in the 
construction of the cells; but it would be a great error to 
suppose that the bees cannot build up a rough wall of wax in 
the proper position — that is, along the plane of intersection 


between two adjoining spheres. I have several specimens 
showing clearly that they can do this. Even in the rude 
circumferential rim or wall of wax round a growing comb, 
flexures may sometimes be observed, corresponding in posi- 
tion to the planes of the rhombic basal plates of future cells. 
But the rough wall of wax has in every case to be finished 
off, by being largely gnawed away on both sides. The 
manner in which the bees build is curious ; they always make 
the first rough wall from ten to twenty times thicker than 
the excessively thin finished wall of the cell, which will 
ultimately be left. We shall understand how they work, by 
supposing masons first to pile up a broad ridge of cement, 
and then to begin cutting it away equally on both sides near 
the ground, till a smooth, very thin wall is left in the middle ; 
the masons always piling up the cut-away cement, and 
adding fresh cement on the summit of the ridge. We shall 
thus have a thin wall steadily growing upward but always 
crowned bv a gigantic coping. From all the cells, both those 
just commenced and those completed, being thus crowned by 
a strong coping of wax, the bees can cluster and crawl over 
the com.b without injuring the delicate hexagonal walls. 
These walls, as Professor Miller has kindly ascertained for 
me, vary greatly in thickness; being, on an average of 
twelve measurements made near the border of the comb, 
T-W of an inch in thickness; whereas the basal rhomboidal 
plates are thicker, nearly in the proportion of three to two, 
having a mean thickness, from twenty-one measurements, 
of ^i^-g of an inch. By the above singular manner of build- 
ing, strength is continually given to the comb, with the ut- 
most ultimate economy of wax. 

It seems at first to add to the difficulty of understanding 
how the cells are made, that a multitude of bees all work 
together ; one bee after working a short time at one cell 
going to another, so that, as Huber has stated, a score of in- 
dividuals work even at the commencement of the first cell. 
I was able practically to show this fact, by covering the 
edges of the hexagonal walls of a single cell, or the extreme 
margin of the circumferential rim of a growing comb, with 
an extremely thin layer of melted vermilion wax; and I in- 
variably found that the colour was most delicately diffused 


by the bees — as delicately as a painter could have done it 
with his brush — by atoms of the coloured wax having been 
taken from the spot on which it had been placed, and worked 
into the growing edges of the cells all round. The work 
of construction seems to be a sort of balance struck between 
many bees, all instinctively standing at the same relative 
distance from each other, all trying to sweep equal spheres, 
and then building up, or leaving ungnawed, the planes of 
intersection between these spheres. It was really curious to 
note in cases of difficulty, as when two pieces of comb met 
at an angle, how often the bees would pull down and rebuild 
in different ways the same cell, sometimes recurring to a 
shape which they had at first rejected. 

When bees have a place on which they can stand in their 
proper positions for working, — for instance, on a slip of 
wood, placed directly under the middle of a comb growing 
downwards, so that the comb has to be built over one face 
of the slip — in this case the bees can lay the foundations 
of one wall of a new hexagon, in its strictly proper place, 
projecting beyond the other completed cells. It suffices that 
the bees should be enabled to stand at their proper relative 
distances from each other and from the walls of the last 
completed cells, and then, by striking imaginary spheres, 
they can build up a wall intermediate between two adjoin- 
ing spheres; but, as far as I have seen, they never gnaw- 
away and finish off the angles of a cell till a large part both 
of that cell and of the adjoining cells has been built. This 
capacity in bees of laying down under certain circumstances 
a rough wall in its proper place between two just-commenced 
cells, is important, as it bears on a fact, which seems at 
first subversive of the foregoing theory; namely, that the 
cells on the extreme margin of wasp-combs are sometimes 
strictly hexagonal; but I have not space here to enter on 
this subject. Nor does there seem to me any great difficulty 
in a single insect (as in the case of a queen-wasp) making 
hexagonal cells, if she were to work alternately on the in- 
side and outside of two or three cells commenced at the 
same time, always standing at the proper relative distance 
from the parts of the cells just begun, sweeping spheres or 
cylinders, and building up intermediate planes. 


As natural selection acts only by the accumulation of 
slight modifications of structure or instinct, each profitable 
to the individual under its conditions of life, it may reason- 
ably be asked, how* a long and graduated succession of modi- 
fied architectural instincts, all tending towards the present 
perfect plan of construction, could have profited the progeni- 
tors of the hive-bee? I think the answer is not difficult: 
cells constructed like those of the bee or the wasp gain in 
strength, and save much in labour and space, and in the ma- 
terials of which they are constructed. With respect to the 
formation of wax, it is known that bees are often hard 
pressed to get sufficient nectar, and I am informed by Mr. 
Tegetmeier that it has been experimentally proved that from 
twelve to fifteen pounds of dry sugar are consumed by a 
hive of bees for the secretion of a pound of wax; so that 
a prodigious quantity of fluid nectar must be collected and 
consumed by the bees in a hive for the secretion of the wax 
necessary for the construction of their combs. Moreover, 
many bees have to remain idle for many days during the 
process of secretion. A large store of honey is indispensable 
to support a large stock of bees during the winter; and the 
security of the hive is known mainly to depend on a large 
number of bees being supported. Hence the saving of wax 
by largely saving honey and the time consumed in collect- 
ing the honey must be an important element of success to 
any family of bees. Of course the success of the species 
may be dependent on the number of its enemies, or parasites, 
or on quite distinct causes, and so be altogether independent 
of the quantity of honey which the bees can collect. But 
let us suppose that this latter circumstance determined, as 
it probably often has determined, whether a bee allied to 
our humble-bees could exist in large numbers in any coun- 
try; and let us further suppose that the community lived 
through the winter, and consequently required a store of 
honey: there can in this case be no doubt that it would be 
an advantage to our imaginary humble-bee, if a slight modi- 
fiication in her instincts led her to make her waxen cells 
near together, so as to intersect a little ; for a wall in com- 
mon even to two adjoining cells would save some little labour 
and wax. Hence it would continually be more and more 


advantageous to our humble-bees, if they were to make their 
cells more and more regular, nearer together, and aggre- 
gated into a mass, like the cells of the Melipona; for in this 
case a large part of the bounding surface of each cell would 
serve to bound the adjoining cells, and much labour and wax 
would be saved. Again, from the same cause, it would be 
advantageous to the Melipona, if she were to make her cells 
closer together, and more regular in every way than at pres- 
ent; for then, as we have seen, the spherical surfaces would 
wholly disappear and be replaced by plane surfaces; and 
the Melipona would make a comb as perfect as that of the 
hive-bee. Beyond this stage of perfection in architecture, 
natural selection could not lead; for the comb of the hive- 
bee, as far as we can see, is absolutely perfect in economis- 
ing labour and wax. 

Thus, as I believe, the most wonderful of all known in- 
stincts, that of the hive-bee, can be explained by natural 
selection having taken advantage of numerous, successive, 
slight modifications of simpler instincts; natural selection 
having, by slow degrees, more and more perfectly led the 
bees to sweep equal spheres at a given distance from each 
other in a double layer, and to build up and excavate the wax 
along the planes of intersection ; the bees, of course, no more 
knowing that they swept their spheres at one particular dis- 
tance from each other, than they know what are the several 
angles of the hexagonal prisms and of the basal rhombic 
plates ; the motive power of the process of natural selection 
having been the construction of cells of due strength and of 
the proper size and shape for the larvae, this being effected 
with the greatest possible economy of labour and wax; that 
individual swarm which thus made the best cells with least 
labour, and least waste of honey in the secretion of wax, 
having succeeded best, and having transmitted their newly 
acquired economical instincts to new swarms, which in their 
turn will have had the best chance of succeeding in the 
struggle for existence. 



■ ' - 1 i 





ft) . 



Facsimile of a page from 

Darwin's notebook 

of 1837 



It has been objected to the foregoing view of the origin of 
instincts that "the variations of structure and of instinct must 
have been simultaneous and accurately adjusted to each other 
as a modification in the one without an immediate correspond- 
ing change in the other would have been fatal." The force 
of this objection rests entirely on the assumption that the 
changes in the instincts and structure are abrupt. To take 
as an illustration the case of the larger titmouse (Parus 
major) alluded to in a previous chapter; this bird often holds 
the seeds of the yew between its feet on a branch, and ham- 
mers with its beak till it gets at the kernel. Now what spe- 
cial diihculty would there be in natural selection preserving 
all the slight individual variations in the shape of the beak, 
which were better and better adapted to break open the seeds, 
until a beak was formed, as well constructed for this purpose 
as that of the nuthatch, at the same time that habit, or com- 
pulsion, or spontaneous variations of taste, led the bird to 
become more and more of a seed-eater? In this case the beak 
is supposed to be slowly modified by natural selection, subse- 
quently to, but in accordance with, slowly changing habits 
or taste ; but let the feet of the titmouse vary and grow larger 
from correlation with the beak, or from any other unknown 
cause, and it is not improbable that such larger feet would 
lead the bird to climb more and more until it acquired the 
remarkable climbing instinct and power of the nuthatch. In 
this case a gradual change of structure is supposed to lead to 
changed instinctive habits. To take < one more case: few 
instincts are more remarkable than that which leads the swift 
of the Eastern Islands to make its nest wholly of inspissated 
saliva. Some birds build their nests of mud, believed to be 
moistened with saliva ; and one of the swifts of North 
America makes its nest (as I have seen) of sticks aggluti- 
nated with saliva, and even with flakes of this substance. Is 
it then very improbable that the natural selection of individual 
swifts, which secreted more and more saliva, should at last 
produce a species with instincts leading it to neglect other 
materials, and to make its nest exclusively of inspissated 



saliva? And so in other cases. It must, however, be admitted 
that in many instances we cannot conjecture whether it was 
instinct or structure which first varied. 

No doubt many instincts of very difficult explanation could 
be opposed to the theory of natural selection — cases, in which 
we cannot see how an instinct could have originated; cases, 
in which no intermediate gradations are known to exist; 
cases of instincts of such trifling importance, that they could 
hardly have been acted on by natural selection; cases of in- 
stincts almost identically the same in animals so remote in 
the scale of nature, that we cannot account for their simi- 
larity by inheritance from a common progenitor, and conse- 
quently must believe that they were independently acquired 
through natural selection. I will not here enter on these 
several cases, but will confine myself to one special difficulty, 
which at first appeared to me insuperable, and actually fatal 
to the whole theory. I allude to the neuters or sterile females 
in insect-communities; for these neuters often differ widely 
in instinct and in structure from both the males and fertile 
females, and yet, from being sterile, they cannot propagate 
their kind. 

The subject well deserves to be discussed at great length, 
but I will here take only a single case, that of working or 
sterile ants. How the workers have been rendered sterile 
is a difficulty; but not much greater than that of any other 
striking modification of structure; for it can be shown that 
some insects and other articulate animals in a state of nature 
occasionally become sterile; and if such insects had been 
social, and it had been profitable to the community that a 
number should have been annually born capable of work, but 
incapable of procreation, I can see no especial difficulty in 
this having been effected through natural selection. But I 
must pass over this preliminary difficulty. The great difficulty 
lies in the working ants differing widely from both the males 
and the fertile females in structure, as in the shape of the 
thorax, and in being destitute of wings and sometimes of 
eyes, and in instinct. As far as instinct alone is concerned, 
the wonderful difference in this respect between the workers 
and the perfect females, would have been better exemplified 
by the hive-bee. If a working ant or other neuter insect had 


been an ordinary animal, I should have unhesitatingly as- 
sumed that all its characters had been slowly acquired through 
natural selection ; namely, by individuals having been born 
with slight profitable modifications, which were inherited by 
the offspring; and that these again varied and again were 
selected, and so onwards. But with the working ant we have 
an insect differing greatly from its parents, yet absc^utely 
sterile, so that it could never have transmitted successively 
acquired modifications of structure or instinct to its progeny. 
It may well be asked how is it possible to reconcile this case 
with the theory of natural selection? 

First, let it be remembered that we have innumerable in- 
stances, both in our domestic productions and in those in a 
state of nature, of all sorts of differences of inherited struc- 
ture which are correlated with certain ages, and with either 
sex. We have differences correlated not only with one sex, 
but with that short period when the reproductive system is 
active, as in the nuptial plumage of many birds, and in the 
hooked jaws of the male salmon. We have even slight dif- 
ferences in the horns of different breeds of cattle in relation 
to an artificially imperfect state of the male sex; for oxen 
of certain breeds have longer horns than the oxen of other 
breeds, relatively to the length of the horns in both the bulls 
and cows of these same breeds. Hence I can see no great 
difficulty in any character becoming correlated with the sterile 
condition of certain members of insect-communities: the dif- 
ficulty lies in understanding how such correlated modifications 
of structure could have been slowly accumulated by natural 

This difficulty, though appearing insuperable, is lessened, 
or, as I believe, disappears, when it is remembered that selec- 
tion may be applied to the family, as well as to the individual, 
and may thus gain the desired end. Breeders of cattle wish 
the flesh and fat to be well marbled together: an animal thus 
characterised has been slaughtered, but the breeder has gone 
with confidence to the same stock and has succeeded. Such 
faith may be placed in the power of selection, that a breed 
of cattle, always yielding oxen with extraordinarily long 
horns, could, it is probable, be formed by carefully watching 
which individual bulls and cows, when matched, produce oxen 


with the longest horns; and yet no one ox would ever have 
propagated its kind. Here is a better and real illustration : 
according to M. Verlot, some varieties of the double annual 
Stock from having been long and carefully selected to the 
right degree, always produce a large proportion of seedlings 
bearing double and quite sterile flowers ; but they likewise 
yield some single and fertile plants. These latter, by which 
alone the variety can be propagated, may be compared with 
the fertile male and female ants, and the double sterile plants 
with the neuters of the same community. As with the varie- 
ties of the stock, so with social insects, selection has been 
applied to the family, and not to the individual, for the sake 
of gaining a serviceable end. Hence we may conclude that 
slight modifications of structure or of instinct, correlated 
with the sterile condition of certain members of the com- 
munity, have proved advantageous : consequently the fertile 
males and females have flourished, and transmitted to their 
fertile offspring a tendency to produce sterile members with 
the same modifications. This process must have been re- 
peated many times, until that prodigious amount of difference 
between the fertile and sterile females of the same species 
has been produced, which we see in many social insects. 

But we have not as yet touched on the acme of the diffi- 
culty ; namely, the fact that the neuters of several ants differ, 
not only from the fertile females and males, but from each 
other, sometimes to an almost incredible degree, and are thus 
divided into two or even three castes. The castes, moreover, 
do not commonly graduate into each other, but are perfectly 
well defined ; being as distinct from each other as are any two 
species of the same genus, or rather as any two genera of the 
same family. Thus in Eciton, there are working and soldier 
neuters, with jaws and instincts extraordinarily different: in 
Cryptocerus, the workers of one caste alone carry a wonder- 
ful sort of shield on their heads, the use of which is quite 
unknown: in the Mexican Myrmecocystus, the workers of 
one caste never leave the nest; they are fed by the workers 
of another caste, and they have an enormously developed ab- 
domen which secretes a sort of honey, supplying the place of 
that excreted by the aphides, or the domestic cattle as they 
may be called, which our European ants guard and imprison. 


It will indeed be thought that I have an overweening con- 
fidence in the principle of natural selection, when I do not 
admit that such wonderful and well-established facts at once 
annihilate the theory. In the simpler case of neuter insects 
all of one caste, which, as I believe, havelbeen rendered dif- 
ferent from the fertile males and females through natural 
selection, we may conclude from the analogy of ordinary 
variations, that the successive, slight, profitable modifications 
did not first arise in all the neuters in the same nest, but in 
some few alone ; and that by the survival of the communities 
with females which produced most neuters having the ad- 
vantageous modification, all the neuters ultimately came to be 
thus characterized. According to this view we ought occa- 
sionally to find in the same nest neuter insects, presenting 
gradations of structure; and this we do find, even not rarely 
considering how few neuter insects out of Europe have been 
carefully examined. Mr. F. Smith has shown that the neuters 
of several British ants dififer surprisingly from each other in 
size and sometimes in colour; and that the extreme forms can 
be linked together by individuals taken out of the same nest: 
I have myself compared perfect gradations of this kind. It 
sometimes happens that the larger or the smaller sized 
workers are the most numerous ; or that both large and small 
are numerous, whilst those of an intermediate size are scanty 
in numbers. Formica flava has larger and smaller workers, 
with some few of intermediate size ; and, in this species, as 
Mr. F. Smith has observed, the larger workers have simple 
eyes (ocelli), which though small can be plainly distinguished, 
whereas the smaller workers have their ocelli rudimentary. 
Having carefully dissected several specimens of these 
workers, I can affirm that the eyes are far more rudi- 
mentary in the smaller workers than can be accounted 
for merely by their proportionally lesser size ; and I fully 
believe, though I dare not assert so positively, that the workers 
of intermediate size have their ocelli in an exactly inter- 
mediate condition. So that here we have two bodies of sterile 
workers in the same nest, differing not only in size, but in 
their organs of vision, yet connected by some few members 
in an intermediate condition. I may digress by adding, that 
if the smaller workers had been the most useful to the com- 


munity, and those males and females had been continually 
selected, which produced more and more of the smaller 
workers, until all the workers were in this condition; we 
should then have had a species of ant with neuters in nearly 
the same condition as those of Myrmica. For the workers of 
Myrmica have not even rudiments of ocelli, though the male 
and female ants of this genus have well-developed ocelli. 

I may give one other case: so confidently did I expect 
occasionally to find gradations of important structures be- 
tween the different castes of neuters in the same species, that 
I gladly availed myself of Mr. F. Smith's offer of numerous 
specimens from the same nest of the driver ant (Anomma) 
of West Africa. The reader will perhaps best appreciate the 
amount of difference in these workers, by my giving not the 
actual measurements, but a strictly accurate illustration: the 
difference was the same as if we were to see a set of work- 
men building a house, of whom many were five feet four 
inches high, and many sixteen feet high; but we must in 
addition suppose that the larger workmen had heads four 
instead of three times as big as those of the smaller men, 
and jaws nearly five times as big. The jaws, moreover, of 
the working ants of the several sizes differed wonderfully in 
shape, and in the form and number of the teeth. But the 
important fact for us is, that, though the workers can be 
grouped into castes of different sizes, yet they graduate in- 
sensibly into each other, as does the widely-different struc- 
ture of their jaws. I speak confidently on this latter point, 
as Sir J. Lubbock made drawings for me, with the camera 
lucida, of the jaws which I dissected from the workers of 
the several sizes. Mr. Bates, in his interesting 'Naturalist on 
the Amazons,' has described analogous cases. 

With these facts before me, I believe that natural selec- 
tion, by acting on the fertile ants or parents, could form a 
species which should regularly produce neuters, all of large 
size with one form of jaw, or all of small size with widely 
different jaws; or lastly, and this is the greatest difficulty, 
one set of workers of one size and structure, and simultane- 
ously another set of workers of a different size and struc- 
ture ; — a graduated series having first been formed, as in the 
case of the driver ant, and then the extreme forms having 


been produced in greater and greater numbers, through the 
survival of the parents which generated them, until none 
with an intermediate structure were produced. 

An analogous explanation has been given by Mr. Wallace, 
of the equally complex case, of certain Malayan Butterflies 
regularly appearing under two or even three distinct female 
forms; and by Fritz Miiller, of certain Brazilian crustaceans 
likewise appearing under two widely distinct male forms. 
But this subject need not here be discussed. 

I have now explained how, as I believe, the wonderful fact 
of two distinctly defined castes of sterile workers existing in 
the same nest, both widely different from each other and from 
their parents, has originated. We can see how useful their 
production may have been to a social community of ants, on 
the same principle that the division of labour is useful to 
civilised man. Ants, however, work by inherited instincts 
and by inherited organs or tools, whilst man works by 
acquired knowledge and manufactured instruments. But I 
must confess, that, with all my faith in natural selection, I 
should never have anticipated that this principle could 
have been efficient in so high a degree, had not the case of 
these neuter insects led me to this conclusion. I have, there- 
fore, discussed this case, at some little but wholly insufficient 
length, in order to show the power of natural selection, and 
likewise because this is by far the most serious special dif- 
ficulty which my theory has encountered. The case, also, is 
very interesting, as it proves that with animals, as with 
plants, any amount of modification may be effected by the 
accumulation of numerous, slight, spontaneous variations, 
which are in any way profitable, without exercise or habit 
having been brought into play. For peculiar habits confined 
to the workers or sterile females, however long they might 
be followed, could not possibly affect the males and fertile 
females, which alone leave descendants. I am surprised that 
no one has hitherto advanced this demonstrative case of 
neuter insects, against the well-known doctrine of inherited 
habit, as advanced by Lamarck. 



I have endeavored in this chapter briefly to show that the 
mental qualities of our domestic animals vary, and that the 
variations are inherited. Still more briefly I have attempted 
to show that instincts vary slightly in a state of nature. No 
one will dispute that instincts are of the highest importance 
to each animal. Therefore there is no real difficulty, under 
changing conditions of life, in natural selection accumulating 
to any extent slight modifications of instinct which are in 
any way useful. In many cases habit or use and disuse have 
probably come into play. I do not pretend that the facts 
given in this chapter strengthen in any great degree my 
theory; but none of the cases of difficulty, to the best of my 
judgment, annihilate it. On the other hand, the fact that 
instincts are not always absolutely perfect and are liable to 
mistakes: — that no instinct can be shown to have been pro- 
duced for the good of other animals, though animals take 
advantage of the instincts of others ; — that the canon in 
natural history, of "Natura non facit saltum," is applicable 
to instincts as well as to corporeal structure, and is plainly 
explicable on the foregoing views, but is otherwise inexplic- 
able, — all tend to corroborate the theory of natural selection. 

This theory is also strengthened by some few other facts in 
regard to instincts ; as by that common case of closely allied, 
but distinct, species, when inhabiting distant parts of the 
world and living under considerably different conditions of 
life, yet often retaining nearly the same instincts. For in- 
stance, we can understand, on the principle of inheritance, 
how it is that the thrush of tropical South America lines its 
nest with mud, in the same peculiar manner as does our 
British thrush ; how it is that the Hornbills of Africa and 
India have the same extraordinary instinct of plastering up 
and imprisoning the females in a hole in a tree, with only a 
small hole left in the plaster through which the males feed 
them and their young when hatched ; how it is that the male 
wrens (Troglodytes) of North America build "cock-nests," 
to roost in, like the males of our Kitty-wrens, — a habit wholly 
unlike that of any other known bird. Finally, it may not be 
a logical deduction, but to my imagination it is far more satis- 


factory to look at such instincts as the young- cuckoo ejecting 
its foster-brothers, — ants making slaves, — the larvae of ichneu- 
monida; feeding within the live bodies of caterpillars, — not 
as specially endowed or created instincts, but as small conse- 
quences of one general law leading to the advancement of all 
organic beings, — namely, multiply, vary, let the strongest live 
and the weakest die. 



Distinction between the sterility of first crosses and of hybrids — 
Sterility various in degree, not universal, affected by close inter- 
breeding, removed by domestication — Laws governing the ster- 
ility of hybrids — Sterility not a special endowment, but incidental 
on other differences, not accumulated by natural selection — 
Causes of the sterility of first crosses and of hybrids — Parallel- 
ism between the effects of changed conditions of life and of 
crossing — Dimorphism and Trimorphism — Fertility of varieties 
when crossed, and of their mongrel offspring not universal — 
Hybrids and mongrels compared independently of their fertility 
— Summary. 

THE view commonly entertained by naturalists is that 
species, when intercrossed, have been specially en- 
dowed with sterility, in order to prevent their con- 
fusion. This view certainly seems at first highly probable, 
for species living together could hardly have been kept dis- 
tinct had they been capable of freely crossing. The subject 
is in many ways important for us, more especially as the 
sterility of species when first crossed, and that of their hybrid 
offspring, cannot have been acquired, as I shall show, by the 
preservation of successive profitable degrees of sterility. It 
is an incidental result of differences in the reproductive sys- 
tems of the parent-species. 

In treating this subject, two classes of facts, to a large 
extent fundamentally different, have generally been con- 
founded; namely, the sterility of species when first crossed, 
and the sterility of the hybrids produced from them. 

Pure species have of course their organs of reproduction 
in a perfect condition, yet when intercrossed they produce 
either few or no offspring. Hybrids, on the other hand, have 
their reproductive organs functionally impotent, as may be 
clearly seen in the state of the male element in both plants 
and animals; though the formative organs themselves are 



perfect in structure, as far as the microscope reveals. In the 
first case the two sexual elements which go to form the 
embryo are perfect; in the second case they are either not at 
all developed, or arc imperfectly developed. This distinction 
is important, when the cause of the sterility, which is common 
to the two cases, has to be considered. The distinction prob- 
ably has been slurred over, owing to the sterility in both cases 
being looked on as a special endowment, beyond the province 
of our reasoning powers. 

The fertility of varieties, that is of the forms known or 
believed to be descended from common parents, when crossed, 
and likewise the fertility of their mongrel offspring, is, with 
reference to my theory, of equal importance with the sterility 
of species ; for it seems to make a broad and clear distinction 
between varieties and species. 

Degrees of Sterility. — First, for the sterility of species 
when crossed and of their hybrid offspring. It is impossible 
to study the several memoirs and works of those two con- 
scientious and admirable observers, Kolreuter and Gartner, 
who almost devoted their lives to this subject, without being 
deeply impressed with the high generality of some degree of 
sterility. Kolreuter makes the rule universal; but then he 
cuts the knot, for in ten cases in which he found two forms, 
considered by most authors as distinct species, quite fertile 
together, he unhesitatingly ranks them as varieties. Gartner, 
also, makes the rule equally universal ; and he disputes the 
entire fertility of Kolreuter's ten cases. But in these and in 
many other cases, Gartner is obliged carefully to count the 
seeds, in order to show that there is any degree of sterility. He 
always compares the maximum number of seeds produced by 
two species when first crossed, and the maximum produced 
by their hybrid offspring, with the average number produced 
by their pure parent-species in a state of nature. But causes 
of serious error here intervene : a plant, to be hybridised, 
must be castrated, and, what is often more important, must 
be secluded in order to prevent pollen being brought to it 
by insects from other plants. Nearly all the plants experi- 
mented on by Gartner were potted, and were kept in a 
chamber in his house. That these processes are often in- 
jurious to the fertility of a plant cannot be doubted; for 


Gartner gives in his table about a score of cases of plants 
which he castrated, and artificially fertilised with their own 
pollen, and (excluding all cases such as the Leguminosae, in 
which there is an acknowledged difficulty in the manipula- 
tion) half of these twenty plants had their fertility in some 
degree impaired. Moreover, as Gartner repeatedly crossed 
some forms, such as the common red and blue pimpernels 
(Anagallis arvensis and cceulea), which the best botanists 
rank as varieties, and found them absolutely sterile, we may 
doubt whether many species are really so sterile, when inter- 
crossed, as he believed. 

It is certain, on the one hand, that the sterility of various 
species when crossed is so different in degree and graduates 
away so insensibly, and, on the other hand, that the fertility 
of pure species is so easily affected by various circumstances, 
that for all practical purposes it is most difficult to say where 
perfect fertility ends and sterility begins. I think no better 
evidence of this can be required than that the two most ex- 
perienced observers who have ever lived, namely Kolreuter 
and Gartner, arrived at diametrically opposite conclusions in 
regard to some of the very same forms. It is also most in- 
structive to compare — but I have not space here to enter into 
details — the evidence advanced by our best botanists on the 
question whether certain doubtful forms should be ranked as 
species or varieties, with the evidence from fertility adduced 
by different hybridisers, or by the same observer from ex- 
periments made during different years. It can thus be shown 
that neither sterility nor fertility affords any certain distinc- 
tion between species and varieties. The evidence from this 
source graduates away, and is doubtful in the same degree as 
is the evidence derived from other constitutional and struc- 
tural differences. 

In regard to the sterility of hybrids in successive genera- 
tions ; though Gartner was enabled to rear some hybrids, care- 
fully guarding them from a cross with either pure parent, for 
six or seven, and in one case for ten generations, yet he 
asserts positively that their fertility never increases, but gen- 
erally decreases greatly and suddenly. With respect to this 
decrease, it may first be noticed that when any deviation in 
structure or constitution is common to both parents, this is 


often transmitted in an augmented degree to the offspring; 
and both sexual elements in hybrid plants are already affected 
in some degree. But I believe that their fertility has been 
diminished in nearly all these cases by an independent cause, 
namely, by too close interbreeding. I have made so many 
experiments and collected so many facts, showing on the one 
hand that an occasional cross with a distinct individual or 
variety increases the vigour and fertility of the offspring, and 
on the other hand that very close interbreeding lessens their 
vigour and fertility, that I cannot doubt the correctness of 
this conclusion. Hybrids are seldom raised by experimental- 
ists in great nurribers ; and as the parent-species, or other 
allied hybrids, generally grow in the same garden, the visits 
of insects must be carefully prevented during the flowering 
season ; hence hybrids, if left to themselves, will generally be 
fertilised during each generation by pollen from the same 
flower; and this would probably be injurious to their fertility, 
already lessened by their hybrid origin. I am strengthened 
in this conviction by a remarkable statement repeatedly made 
by Gartner, namely, that if even the less fertile hybrids be 
artificially fertilised with hybrid pollen of the same kind, their 
fertility, nothwithstanding the frequent ill effects from manip- 
ulation, sometimes decidedly increases, and goes on increas- 
ing. Now, in the process of artificial fertilisation, pollen is 
as often taken by chance (as I know from my own experi- 
ence) from the anthers of another flower, as from the anthers 
of the flower itself which is to be fertilised; so that a cross 
between two flowers, though probably often on the same 
plant, would be thus effected. Moreover, whenever compli- 
cated experiments are in progress, so careful an observer as 
Gartner would have castrated his hybrids, and this would 
have ensured in each generation a cross with pollen from 
a distinct flower, either from the same plant or from another 
plant of the same hybrid nature. And thus, the strange fact 
of an increase of fertility in the successive generations of 
artificially fertilised hybrids, in contrast with those spon- 
taneously self-fertilised, may, as I believe, be accounted for 
by too close interbreeding having been avoided. 

Now let us turn to the results arrived at by a third most 
experienced hybridiscr, namely, the Hon. and Rev. W. Iler- 


bert. He is as emphatic in his conclusion that some hybrids 
are perfectly fertile — as fertile as the pure parent-species — 
as are Kolreuter and Gartner that some degree of sterility 
between distinct species is a universal law of nature. He 
experimented on some of the very same species as did 
Gartner. The difference in their results may, I think, be in 
part accounted for by Herbert's great horticultural skill, and 
by his having hot-houses at his command. Of his many im- 
portant statements I will here give only a single one as an 
example, namely, that "every ovule in a pod of Crinum 
capense fertilised by C. revolutum produced a plant, which 
I never saw to occur in a case of its natural fecundation." 
So that here we have perfect or even more than com- 
monly perfect fertility, in a first cross between two distinct 

This case of the Crinum leads me to refer to a singular 
fact, namely, that individual plants of certain species of 
Lobelia, Verbascum and Passiflora, can easily be fertilised by 
pollen from a distinct species, but not by pollen from the 
same plant, though this pollen can be proved to be perfectly 
sound by fertilising other plants or species. In the genus 
Hippeastrum, in Corydalis as shown by Professor Hilde- 
brand, in various orchids as shown by Mr. Scott and Fritz 
Miiller, all the individuals are in this peculiar condition. So 
that with some species, certain abnormal individuals, and in 
other species all the individuals, can actually be hybridised 
much more readily than they can be fertilised by pollen from 
the same individual plant ! To give one instance, a bulb of 
Hippeastrum aulicum produced four flowers ; three were fer- 
tilised by Herbert with their own pollen, and the fourth was 
subsequently fertilised by the pollen of a compound hybrid 
descended from three distinct species : the result was that 
"the ovaries of the three first flowers soon ceased to grow, 
and after a few days perished entirely, whereas the pod im- 
pregnated by the pollen of the hybrid made vigorous growth 
and rapid progress to maturity, and bore good seed, which 
vegetated freely." Mr. Herbert tried similar experiments 
during many years, and always with the same result. These 
cases serve to show on what slight and mysterious causes the 
lesser or greater fertility of a species sometimes depends. 


The practical experiments of horticulturists, though not 
made with scientific precision, deserve some notice. It is 
notorious in how complicated a manner the species of Pelar- 
gonium, Fuchsia, Calceolaria, Petunia, Rhododendron, &c., 
have been crossed, yet many of these hybrids seed freely. 
For instance, Herbert asserts that a hybrid from Calceolaria 
integrifolia and plantaginea, species most widely dissimilar 
in general habit, "reproduces itself as perfectly as if it had 
been a natural species from the mountains of Chili." I have 
taken some pains to ascertain the degree of fertility of some 
of the complex crosses of Rhododendrons, and I am assured 
that many of them are perfectly fertile. Mr. C. Noble, for 
instance, informs me that he raises stocks for grafting from 
a hybrid between Rhod. ponticum and catawbiense, and that 
this hybrid "seeds as freely as it is possible to imagine." Had 
hybrids, when fairly treated, always gone on decreasing in 
fertility in each successive generation, as Gartner believed 
to be the case, the fact would have been notorious to nursery- 
men. Horticulturists raise large beds of the same hybrid, and 
such alone are fairly treated, for by insect-agency the several 
individuals are allowed to cross freely with each other, and 
the injurious influence of close interbreeding is thus pre- 
vented. Any one may readily convince himself of the effici- 
ency of insect-agency by examining the flowers of the more 
sterile kinds of hybrid Rhododendrons, which produce no 
pollen, for he will find on their stigmas plenty of pollen 
brought from other flowers. 

In regard to animals, much fewer experiments have been 
carefully tried than with plants. If our systematic arrange- 
ments can be trusted, that is, if the genera of animals are as 
distinct from each other as are the genera of plants, then 
we may infer that animals more widely distinct in the scale 
of nature can be crossed more easily than in the case of 
plants ; but the hybrids themselves are, I think, more sterile. 
It should, however, be borne in mind that, owing to few 
animals breeding freely under confinement, few experiments 
have been fairly tried : for instance, the canary-bird has been 
crossed with nine distinct species of finches, but, as not one 
of these breeds freely in confinement, we have no right to 
expect that the first crosses between them and the canary, 


or that their hybrids, should be perfectly fertile. Again, with 
respect to the fertility in successive generations of the more 
fertile hybrid animals, I hardly know of an instance in which 
two families of the same hybrid have been raised at the same 
time from different parents, so as to avoid the ill effects of 
close interbreeding. On the contrary, brothers and sisters 
have usually been crossed in each successive generation, in 
opposition to the constantly repeated admonition of every 
breeder. And in this case, it is not. at all surprising that the 
inherent sterility in the hybrids should have gone on in- 

Although I know of hardly any thoroughly well-authen- 
ticated cases of perfectly fertile hybrid animals, I have reason 
to believe that the hybrids from Cervulus vaginalis and Ree- 
vesii, and from Phasianus colchicus with P. torquatus, are 
perfectly fertile. M. Quatrefages states that the hybrids from 
two moths (Bombyx cynthia and arrindia) were proved in 
Paris to be fertile inter se for eight generations. It has lately 
been asserted that two such distinct species as the hare and 
rabbit, when they can be got to breed together, produce ofif- 
spring, which are highly fertile when crossed with one of 
the parent-species. The hybrids from the common and Chi- 
nese geese (A. cygnoides), species which are so different that 
they are generally ranked in distinct genera, have often bred 
in this country with either pure parent, and in one single in- 
stance they have bred inter se. This was effected by Mr. 
Eyton, who raised two hybrids from the same parents, but 
from different hatches ; and from these two birds he raised 
no less than eight hybrids (grandchildren of the pure geese) 
from one nest. In India, however, these cross-bred geese 
must be far more fertile ; for I am assured by two eminently 
capable judges, namely Mr. Blyth and Capt. Hutton, that 
whole flocks of these crossed geese are kept in various parts 
of the country ; and as they are kept for profit, where neither 
pure parent-species exists, they must certainly be highly or 
perfectly fertile. 

With our domesticated animals, the various races when 
crossed together are quite fertile ; yet in many cases they are 
descended from two or more wild species. From this fact we 
must conclude either that the aboriginal parent-species at 


first produced perfectly fertile hybrids, or that the hybrids 
subsequently reared under domestication became quite fertile. 
This latter alternative, which was first propounded by Pallas, 
seems by far the most probable, and can, indeed, hardly be 
doubted. It is, for instance, almost certain that our dogs are 
descended from several wild stocks; yet, with perhaps the 
exception of certain indigenous domestic dogs of South 
America, all are quite fertile together; but analogy makes 
me greatly doubt, whether the several aboriginal species would 
at first have freely bred together and have produced quite 
fertile hybrids. So again I have lately acquired decisive evi- 
dence that the crossed offspring from the Indian humped and 
common cattle are inter se perfectly fertile; and from the 
observations by Riitimeyer on their important osteological 
differences, as well as from those by Mr. Bl}'lh on their dif- 
ferences in habits, voice, constitution, &c., these two forms 
must be regarded as good and distinct species. The same re- 
marks may be extended to the two chief races of the pig. 
We must, therefore, either give up the belief of the universal 
sterility of species when crossed; or we must look at this 
sterility in animals, not as an indelible characteristic, but as 
one capable of being removed by domestication. 

Finally, considering all the ascertained facts on the inter- 
crossing of plants and animals, it may be concluded that some 
degree of sterility, both in first crosses and in hybrids, is an 
extremely general result ; but that it cannot, under our present 
state of knowledge, be considered as absolutely universal. 



We will now consider a little more in detail the laws gov- 
erning the sterility of first crosses and of hybrids. Our chief 
object will be to see whether or not these laws indicate that 
species have been specially endowed with this quality, in order 
to prevent their crossing and blending together in utter con- 
fusion. The following conclusions are drawn up chiefly from 
Gartner's admirable work on the hybridisation of plants. I 
have taken much pains to ascertain how far they apply to 
animals, and, considering how scanty our knowledge is in re- 


gard to hybrid animals, I have been surprised to find how 
generally the same rules apply to both kingdoms. 

It has been already remarked, that the degree of fertility, 
both of first crosses and of hybrids, graduates from zero to- 
perfect fertility. It is surprising in how many curious ways 
this gradation can be shown; but only the barest outline of 
the facts can here be given. When pollen from a plant of 
one family is placed on the stigma of a plant of a distinct 
family, it exerts no more influence than so much inorganic 
dust. From this absolute zero of fertility, the pollen of dif- 
ferent species applied to the stigma of some one species of 
the same genus, yields a perfect gradation in the number of 
seeds produced, up to nearly complete or even quite complete 
fertility; and, as we have seen, in certain abnormal cases, 
even to an excess of fertility, beyond that which the plant's 
own pollen produces. So in hybrids themselves, there are 
some which never have produced, and probably never would 
produce, even with the pollen of the pure parents, a single 
fertile seed: but in some of these cases a first trace of fer- 
tility may be detected, by the pollen of one of the pure parent- 
species causing the flower of the hybrid to wither earlier 
than it otherwise would have done; and the early withering 
of the flower is well known to be a sign of incipient fertilisa- 
tion. From this extreme degree of sterility we have self- 
fertilised hybrids producing a greater and greater number of 
seeds up to perfect fertility. 

The hybrids raised from two species which are very diifi- 
cult to cross, and which rarely produce any offspring, are 
generally very sterile; but the parallelism between the diffi- 
culty of making a first cross, and the sterility of the hybrids 
thus produced — two classes of facts which are generally con- 
founded together — is by no means strict. There are many 
cases, in which two pure species, as in the genus Verbascum, 
can be united with unusual facility, and produce numerous 
hybrid-offspring, yet these hybrids are remarkably sterile. 
On the other hand, there are species which can be crossed 
very rarely, or with extreme difficulty, but the hybrids, when 
at last produced, are very fertile. Even within the limits of 
the same genus, for instance in Dianthus, these two opposite 
cases occur. 


The fertility, both of first crosses and of hybrids, is more 
easily affected by unfavorable conditions, than is that of 
pure species. But the fertility of first crosses is likewise in- 
nately variable ; for it is not always the same in degree when 
the same two species are crossed under the same circum- 
stances ; it depends in part upon the constitution of the in- 
dividuals which happen to have been chosen for the experi- 
ment. So it is with hybrids, for their degree of fertility is 
often found to differ greatly in the several individuals raised 
from seed out of the same capsule and exposed to the same 

By the term systematic affinity is meant, the general re- 
semblance between species in structure and constitution. Now 
the fertility of first crosses, and of the hybrids produced from 
them, is largely governed by their systematic affinity. This 
is clearly shown by hybrids never having been raised between 
species ranked by systematists in distinct families; and on 
the other hand, by very closely allied species generally uniting 
with facility. But the correspondence between systematic 
affinity and the facility of crossing is by no means strict. A 
multitude of cases could be given of very closely allied species 
which will not unite, or only with extreme difficulty ; and on 
the other hand of very distinct species which unite with the 
utmost facility. In the same family there may be a genus, 
as Dianthus, in which very man}^ species can most readily be 
crossed ; and another genus, as Silene, in which the most per- 
severing efforts have failed to produce between extremely 
close species a single hybrid. Even within the limits of the 
same genus, we meet with this same difference ; for instance, 
the many species of Nicotiana have been more largely crossed 
than the species of almost any other genus ; but Gartner 
found that N. acuminata, which is not a particularly distinct 
species, obstinately failed to fertilise, or to be fertilised by no 
less than eight other species of Nicotiana. Many analogous 
facts could be given. 

No one has been able to point out what kind or what 
amount of difference, in any recognisable character, is suf- 
ficient to prevent two species crossing. It can be shown that 
plants most widely different in habit and general appearance, 
and having strongly marked differences in every part of the 


flower, even in the pollen, in the fruit, and in the cotyledons, 
can be crossed. Annual and perennial plants, deciduous and 
evergreen trees, plants inhabiting different stations and fitted 
for extremely different climates, can often be crossed with 

By a reciprocal cross between two species, I mean the case, 
for instance, of a female-ass being first crossed by a stallion, 
and then a mare by a male-ass; these two species may then 
be said to have been reciprocally crossed. There is often the 
widest possible difference in the facility of making reciprocal 
crosses. Such cases are highly important, for they prove 
that the capacity in any two species to cross is often com- 
pletely independent of their systematic affinity, that is of any 
difference in their structure or constitution, excepting in 
their reproductive systems. The diversity of the result in 
reciprocal crosses between the same two species was long 
ago observed by Kolreuter. To give an instance: Mirabilis 
jalapa can easily be fertilised by the pollen of M. longiflora, 
and the hybrids thus produced are sufficiently fertile ; but 
Kolreuter tried more than two hundred times, during eight 
following years, to fertilise reciprocally M. longiflora with 
the pollen of M. jalapa, and utterly failed. Several other 
equally striking cases could be given. Thuret has observed 
the same fact with certain sea-weeds or Fuci. Gartner, 
moreover, found that this difference of facility in making 
reciprocal crosses is extremely common in a lesser degree. 
He has observed it even between closely related forms (as 
Matthiola annua and glabra) which many botanists rank only 
as varieties. It is also a remarkable fact, that hybrids raised 
from reciprocal crosses, though of course compounded of the 
very same two species, the one species having first been used 
as the father and then as the mother, though they rarely 
differ in external characters, yet generally differ in fertility 
in a small, and occasionally in a high degree. 

Several other singular rules could be given from Gartner: 
for instance, some species have a remarkable power of cross- 
ing with other species; other species of the same genus have 
a remarkable power of impressing their likeness on their 
hybrid offspring; but these two powers do not at all neces- 
sarily go together. There are certain hybrids which, instead 


of having, as is usual, an intermediate character between their 
two parents, always closely resemble one of them; and such 
hybrids, though externally so like one of their pure parent- 
species, are with rare exceptions e:xtremely sterile. So again 
amongst hybrids which are usually intermediate in structure 
between their parents, exceptional and abnormal individuals 
sometimes are born, which closely resemble one of their pure 
parents ; and these hybrids are almost always utterly sterile, 
even when the other hybrids raised from seed from the same 
capsule have a considerable degree of fertility. These facts 
show how completely the fertility of a hybrid may be inde- 
pendent of its external resemblance to either pure parent. 

Considering the several rules now given, which govern the 
fertility of first crosses and of hybrids, we see that when 
forms, which must be considered as good and distinct species, 
are united, their fertility graduates from zero to perfect fer- 
tility, or even to fertility under certain conditions in excess; 
that their fertility, besides being eminently susceptible to 
favourable and unfavourable conditions, is innately variable; 
that it is by no means always the same in degree in the first 
cross and in the hybrids produced from this cross ; that the 
fertility of hybrids is not related to the degree in which they 
resemble in external appearance either parent; and lastly, 
that the facility of making a first cross between any two 
species is not always governed by their systematic affinity or 
degree of resemblance to each other. This latter statement 
is clearly proved by the difference in the result of reciprocal 
crosses between the same two species, for, according as the 
one species or the other is used as the father or the mother, 
there is generally some difference, and occasionally the widest 
possible difference, in the facility of effecting an union. The 
hybrids, moreover, produced from reciprocal crosses often 
differ in fertility. 

Now do these complex and singular rules indicate that 
species have been endowed with sterility simply to prevent 
their becoming confounded in nature? I think not. For 
why should the sterility be so extremely different in degree, 
when various species are crossed, all of which we must sup- 
pose it would be equally important to keep from blending to- 
gether? Why should the degree of sterility be innately vari- 


able in the individuals of the same species? Why should 
some species cross with facility, and yet produce very sterile 
hybrids; and other species cross with extreme difficulty, and 
yet produce fairly fertile hybrids? Why should there often 
be so great a difference in the result of a reciprocal cross 
between the same two species ? Why, it may even be asked, 
has the production of hybrids been permitted? To grant to 
species the special power of producing hybrids, and then to 
stop their further propagation by different degrees of sterility, 
not strictly related to the facility of the first union between 
their parents, seems a strange arrangement. 

The foregoing rules and facts, on the other hand, appear to 
me clearly to indicate that the sterility both of first crosses 
and of hybrids is simply incidental or dependent on unknown 
differences in their reproductive systems; the differences be- 
ing of so peculiar and limited a nature, that, in reciprocal 
crosses between the same two species, the male sexual ele- 
ment of the one will often freely act on the female sexual 
element of the other, but not in a reversed direction. It will 
be advisable to explain a little more fully by an example what 
I mean by sterility being incidental on other differences, and 
not a specially endowed quality. As the capacity of one 
plant to be grafted or budded on another is unimportant for 
their welfare in a state of nature, I presume that no one will 
suppose that this capacity is a specially endowed quality, but 
•will admit that it is incidental on differences in the laws of 
growth of the two plants. We can sometimes see the reason 
why one tree will not take on another, from differences in 
their rate of growth, in the hardness of their wood, in the 
period of the flow or nature of their sap, &c. ; but in a multi- 
tude of cases we can assign no reason whatever. Great di- 
versity in the size of two plants, one being woody and the other 
herbaceous, one being evergreen and the other decidu- 
ous, an adaptation to widely different climates, do not 
always prevent the two grafting together. As in hybridisa- 
tion, so with grafting, the capacity is limited by systematic 
affinity, for no one has been able to graft together trees be- 
longing to quite distinct families; and, on the other hand, 
closely allied species, and varieties of the same species, can 
usually, but not invariably, be grafted with ease. But this 


capacity, as in hybridisation, is by no means absolutely gov- 
erned by systematic affinity. Although many distinct genera 
within the same family have been grafted together, in other 
cases species of the same genus will not take on each other. 
The pear can be grafted far more readily on the quince, 
which is ranked as a distinct genus, than on the apple, which 
is a member of the same genus. Even different varieties of 
the pear take with different degrees of facility on the quince ; 
so do different varieties of the apricot and peach on certain 
varieties of the plum. 

As Gartner found that there was sometimes an innate dif- 
ference in different individuals of the same two species in 
crossing: so Sageret believes this to be the case with different 
individuals of the same two species in being grafted together. 
As in reciprocal crosses, the facility of effecting an union is 
often very far from equal, so it sometimes is in grafting; the 
common gooseberry, for instance, cannot be grafted on the 
currant, whereas the currant will take, though with difficulty, 
on the gooseberry. 

We have seen that the sterility of hybrids, which have 
their reproductive organs in an imperfect condition, is a dif- 
ferent case from the difficulty of uniting two pure species, 
which have their reproductive organs perfect; yet these two 
distinct classes of cases run to a large extent parallel. Some- 
thing analogous occurs in grafting; for Thouin found that 
three species of Robinia, which seeded freely on their own 
roots, and which could be grafted with no great difficulty on 
a fourth species, when thus grafted were rendered barren. 
On the other hand, certain species of Sorbus, when grafted 
on other species yielded twice as much fruit as when on their 
own roots. We are reminded by this latter fact of the extra- 
ordinary cases of Hippeastrum, Passiflora, &c., which seed 
much more freely when fertilised with the pollen of a dis- 
tinct species, than when fertilised with pollen from the same 

We thus see, that, although there is a clear and great dif- 
ference between the mere adhesion of grafted stocks, and the 
union of the male and female elements in the act of repro- 
duction, yet that there is a rude degree of parallelism in the 
results of grafting and of crossing distinct species. And as 


we must look at the curious and complex laws governing the 
facility with which trees can be grafted on each other as in- 
cidental on unknown differences in their vegetative systems, 
so I believe that the still more complex laws governing the 
facility of first crosses are incidental on unknown differences 
in their reproductive systems. These differences in both 
cases, follow to a certain extent, as might have been expected, 
systematic affinity, by which term every kind of resemblance 
and dissimilarity between organic beings is attempted to be 
expressed. The facts by no means seem to indicate that the 
greater or lesser difficulty of either grafting or crossing vari- 
ous species has been a special endowment; although in the 
case of crossing, the difficulty is as important for the endur- 
ance and stability of specific forms, as in the case of graft- 
ing it is unimportant for their welfare. 



At one time it appeared to me probable, as it has to others, 
that the sterility of first crosses and of hybrids might have 
been slowly acquired through the natural selection of slightly 
lessened degrees of fertility, which, like any other variation, 
spontaneously appeared in certain individuals of one variety 
when crossed with those of another variety. For it would 
clearly be advantageous to two varieties or incipient species, 
if they could be kept from blending, on the same principle 
that, when man is selecting at the same time two varieties, 
it is necessary that he should keep them separate. In the 
first place, it may be remarked that species inhabiting dis- 
tinct regions are often sterile when crossed; now it could 
clearly have been of no advantage to such separated species 
to have been rendered mutually sterile, and consequently this 
could not have been effected through natural selection ; but 
it may perhaps be argued, that, if a species was rendered 
sterile with some one compatriot, sterility with other species 
would follow as a necessary contingency. In the second 
place, it is almost as much opposed to the theory of natural 
selection as to that of special creation, that in reciprocal 
crosses the male element of one form should have been ten- 


dered utterly impotent on a second form, whilst at the same 
time the male element of this second form is enabled freely 
to fertilise the first form ; for this peculiar state of the repro- 
ductive system could hardly have been advantageous to either 

In considering the probability of natural selection having 
come into action, in rendering species mutually sterile, the 
greatest diflficulty will be found to lie in the existence of many 
graduated steps from slightly lessened fertility to absolute 
sterility. It may be admitted that it would profit an incipient 
species, if it were rendered in some slight degree sterile when 
crossed with its parent form or with some other variety; for 
thus fewer bastardised and deteriorated offspring would be 
produced to commingle their blood with the new species in 
process of formation. But he who will take the trouble to 
reflect on the steps by which this first degree of sterility 
could be increased through natural selection to that high de- 
gree which is common with so many species, and which is 
universal with species which have been differentiated to a 
generic or family rank, will find the subject extraordinarily 
complex. After mature reflection it seems to me that this 
could not have been effected through natural selection. Take 
the case of any two species which, when crossed, produced 
few and sterile offspring; now, what is there which could 
favour the survival of those individuals which happened to 
be endowed in a slightly higher degree with mutual infer- 
tility, and which thus approached by one small step towards 
absolute sterility? Yet an advance of this kind, if the theory 
of natural selection be brought to bear, must have incessantly 
occurred with many species, for a multitude are mutually 
quite barren. With sterile neuter insects we have reason to 
believe that modifications in their structure and fertility 
have been slowly accumulated by natural selection, from an 
advantage having been thus indirectly given to the com- 
munity to which they belonged over other communities of the 
same species; but an individual animal not belonging to a 
social community, if rendered slightly sterile when crossed 
with some other variety, would not thus itself gain any ad- 
vantage or indirectly give any advantage to the other individ- 
uals of the same variety, thus leading to their preservation. 


But it would be superfluous to discuss this question in de- 
tail; for with plants we have conclusive evidence that the 
sterility of crossed species must be due to some principle, 
quite independent of natural selection. Both Gartner and 
Kolreuter have proved that in genera including numerous 
species, a series can be formed from species which when 
crossed yield fewer and fewer seeds, to species which never 
produce a single seed, but yet are affected by the pollen of 
certain other species, for the germen swells. It is here mani- 
festly impossible to select the more sterile individuals, which 
have already ceased to yield seeds ; so that this acme of ster- 
ility, when the germen alone is affected, cannot have been 
gained through selection; and from the laws governing the 
various grades of sterility being so uniform throughout the 
animal and vegetable kingdoms, we may infer that the cause, 
whatever it may be, is the same or nearly the same in all 

We will now look a little closer at the probable nature of 
the differences between species which induce sterility in first 
crosses and in hybrids. In the case of first crosses, the 
greater or less difficulty in effecting an union and in obtain- 
ing offspring apparently depends on several distinct causes. 
There must sometimes be a physical impossibility in the male 
element reaching the ovule, as would be the case with a plant 
having a pistil too long for the pollen-tubes to reach the 
ovarium. It has also been observed that when the pollen of 
one species is placed on the stigma of a distantly allied spe- 
cies, though the pollen-tubes protrude, they do not penetrate 
the stigmatic surface. Again, the male element may reach the 
female element but be incapable of causing an embryo to be 
developed, as seems to have been the case with some of Thu- 
ret's experiments on Fuci. No explanation can be given of 
these facts, any more than why certain trees cannot be grafted 
on others. Lastly an embryo may be developed, and then perish 
at an early period. This latter alternative has not been suf- 
ficiently attended to; but I believe, from observations com- 
municated to me by Mr. Hewitt, who has had great experi- 
ence in hybridising pheasants and fowls, that the early death 
of the embryo is a very frequent cause of sterility in first 


crosses. Mr. Salter has recently given the results of an ex- 
amination of about 500 eggs produced from various crosses 
between three species of Gallus and their hybrids; the ma- 
jority of these eggs had been fertilised; and in the majority 
of the fertilised eggs, the embryos had either been partially 
developed and had then perished, or had become nearly ma- 
ture, but the young chickens had been unable to break through 
the shells. Of the chickens which were born, more than four- 
fifths died within the first few days, or at latest weeks, "with- 
out any obvious cause, apparently from mere inability to 
live;" so that from the 500 eggs only twelve chickens were 
reared. With plants, hybridised embpyos probably often 
perish in a like manner; at least it is known that hybrids 
raised from very distinct species are sometimes weak and 
dwarfed, and perish at an early age; of which fact Max 
Wichura has recently given some striking cases with hybrid 
willows. It may be here worth noticing that in some cases of 
parthenogenesis, the embryos within the eggs of silk moths 
which had not been fertilised, pass through their early stages 
of development and then perish like the embryos produced by 
a cross between distinct species. Until becoming acquainted 
with these facts, I was unwilling to believe in the frequent 
early death of hybrid embryos; for hybrids, when once born, 
are generally healthy and long-lived, as we see in the case 
of the common mule. Hybrids, however, are differently cir- 
cumstanced before and after birth ; when born and living in 
a country where their two parents live, they are generally 
placed under suitable conditions of life. But a hybrid par- 
takes of only half of the nature and constitution of its 
mother ; it may therefore before birth, as long as it is nour- 
ished within its mother's womb, or within the egg or seed 
produced by the mother, be exposed to conditions in some de- 
gree unsuitable, and consequently be liable to perish at an early 
period; more especially as all very young beings are eminently 
sensitive to injurious or unnatural conditions of life. But af- 
ter all, the cause more probably lies in some imperfection 
in the original act of impregnation, causing the embryo to be 
imperfectly developed, rather than in the conditions to which 
it is subsequently exposed. 

In regard to the sterility of hybrids, in which the sexual 


elements are imperfectly developed, the case is somewhat dif- 
ferent. I have more than once alluded to a large body of 
facts showing that, when animals and plants are removed 
from their natural conditions, they are extremely liable to 
have their reproductive systems seriously affected. This, in 
fact, is the great bar to the domestication of animals. Be- 
tween the sterility thus superinduced and that of hybrids, 
there are many points of similarity. In both cases the steril- 
ity is independent of general health, and is often accompanied 
by excess of size or great luxuriance. In both cases the 
sterility occurs in various degrees; in both, the male element 
is the most liable to be affected; but sometimes the female 
more than the male. In both, the tendency goes to a certain 
extent with systematic affinity, for whole groups of animals 
and plants are rendered impotent by the same unnatural con- 
ditions; and whole groups of species tend to produce sterile 
hybrids. On the other hand, one species in a group will some- 
times resist great changes of conditions with unimpaired 
fertility ; and certain species in a group will produce unusually 
fertile hybrids. No one can tell, till he tries, whether any 
particular animal will breed under confinement, or any exotic 
plant seed freely under culture ; nor can he tell till he tries, 
whether any two species of a genus will produce more or 
less sterile hybrids. Lastly, when organic beings are placed 
during several generations under conditions not natural to 
them, they are extremely liable to vary, which seems to be 
partly due to their reproductive systems having been specially 
affected, though in a lesser degree than when sterility ensues. 
So it is with hybrids, for their offspring in successive genera- 
tions are eminently liable to vary, as every experimentalist 
has observed. 

Thus we see that when organic beings are placed under new 
and unnatural conditions, and when hybrids are produced 
by the unnatural crossing of two species, the reproductive 
system, independently of the general state of health, is af- 
fected in a very similar manner. In the one case, the condi- 
tions of life have been disturbed, though often in so slight' 
a degree as to be inappreciable by us ; in the other case, or 
that of hybrids, the external conditions have remained the 
same, but the organisation has been disturbed by two dis- 


tinct structures and constitutions, including of course the 
reproductive systems, having been blended into one. For 
it is scarcely possible that two organisations should be 
compounded into one, without some disturbance occur- 
ring in the development, or periodical action, or mutual 
relations of the different parts and organs one to another or 
to the conditions of life. When hybrids are able to breed 
inter se, they transmit to their offspring from generation to 
generation the same compounded organisation, and hence we 
need not be surprised that their sterility, though in some 
degree variable, does not dim.inish ; it is even apt to increase, 
this being generally the result, as before explained, of too 
close interbreeding. The above view of the sterility of hy- 
brids being caused by two constitutions being compounded 
into one has been strongly maintained by Max Wichura. 

It must, however, be owned that we cannot understand, on 
the above or any other view, several facts with respect to the 
sterility of hybrids ; for instance, the unequal fertility of hy- 
brids produced from reciprocal crosses ; or the increased ster- 
ility in those hybrids which occasionally and exceptionally 
resemble closely either pure parent. Nor do I pretend that 
the foregoing remarks go to the root of the matter ; no ex- 
planation is offered why an organism, when placed under nat- 
ural conditions, is rendered sterile. All that I have attempted 
to show is, that in two cases, in some respects allied, sterility 
is the common result, — in the one case from the conditions 
of life having been disturbed, in the other case from the 
organisation having been disturbed by two organisations 
being compounded into one. 

A similar parallelism holds good with an allied yet very dif- 
ferent class of facts. It is an old and almost universal be- 
lief founded on a considerable body erf evidence, which I have 
elsewhere given, that slight changes in the conditions of life 
are beneficial to all living things. We see this acted on by 
farmers and gardeners in their frequent exchanges of seed, 
tubers, &c., from one soil or climate to another, and back 
again. During the convalescence of animals, great benefit 
is derived from almost any change in their habits of life. 
Again, both with plants and animals, there is the clearest 
evidence that a cross between individuals of the same spe- 


cies, which differ to a certain extent, gives vigour and fer- 
tility to the offspring; and that close interbreeding continued 
during several generations between the nearest relations, if 
these be kept under the same conditions of life, almost always 
leads to decreased size, weakness, or sterility. 

Hence it seems that, on the one hand, slight changes in the 
conditions of life benefit all organic beings, and on the other 
hand, that slight crosses, that is crosses between the males 
and females of the same species, which have been subjected 
to slightly different conditions, or which have slightly varied, 
give vigour and fertility to the offspring. But, as we have 
seen, organic beings long habituated to certain uniform condi- 
tions under a state of nature, when subjected, as under con- 
finement, to a considerable change in their conditions, very 
frequently are rendered more or less sterile ; and we know 
that a cross between two forms, that have become widely or 
specifically different, produce hybrids which are almost al- 
ways in some degree sterile. I am fully persuaded that this 
double parallelism is by no means an accident or an illusion. 
He who is able to explain why the elephant and a multitude 
of other animals are incapable of breeding when kept under 
only partial confinement in their native country, will be 
able to explain the primary cause of hybrids being so gener- 
ally sterile. He will at the same time be able to explain 
how it is that the races of some of our domesticated animals, 
which have often been subjected to new and not uniform con- 
ditions, are quite fertile together, although they are descended 
from distinct species, which would probably have been sterile 
if aboriginally crossed. The above two parallel series of 
facts seem to be connected together by some common but 
unknown bond, which is essentially related to the principle of 
life ; this principle, according to Mr. Herbert Spencer, being 
that life depends on, or consists in, the incessant action and 
reaction of various forces, which, as throughout nature, are 
always tending towards an equilibrium; and when this ten- 
dency is slightly disturbed by any change, the vital forces 
gain in power. 



This subject may be here briefly discussed, and will be 
found to throw some light on hybridism. Several plants be- 
longing to distinct orders present two forms, which exist 
in about equal numbers and which differ in no respect ex- 
cept in their reproductive organs; one form having a long 
pistil with short stamens, the other a short pistil with long 
stamens ; the two having differently sized pollen-grains. 
With trimorphic plants there are three forms likewise differ- 
ing in the lengths of their pistils and stamens, in the size 
and colour of the pollen-grains, and in some other respects; 
and as in each of the three forms there are two sets of sta- 
mens, the three forms possess altogether six sets of stamens 
and three kinds of pistils. These organs are so proportioned 
in length to each other, that half the stamens in two of the 
forms stand on a level with the stigma of the third form. 
Now I have shown, and the result has been confirmed by 
other observers, that, in order to obtain full fertility with 
these plants, it is necessary that the stigma of the one form 
should be fertilised by pollen taken from the stamens of cor- 
responding height in another form. So that with dimorphic 
species two unions, which may be called legitimate, are 
fully fertile; and two, which may be called illegitimate, 
are more or less infertile. With trimorphic species six 
unions are legitimate, or fully fertile, — and twelve are ille- 
gitimate, or more or less infertile. 

The infertility which may be observed in various dimorphic 
and trimorphic plants, when they are illegitimately fertilised, 
that is by pollen taken from stamens not corresponding in 
height with the pistil, differs much in degree, up to absolute 
and utter sterility; just in the same manner as occurs in 
crossing distinct species. As the degree of sterility in the 
latter case depends in an eminent degree on the conditions 
of life being more or less favourable, so I have found it 
with illegitimate unions. It is well known that if pollen of a 
distinct species be placed on the stigma of a flower, and its 
own pollen be afterwards, even after a considerable interval 
of time, placed on the same stigma, its action is so strongly 
prepotent that it generally annihilates the effect of the foreign 


pollen; so it is with the pollen of the several forms of the 
same species, for legitimate pollen is strongly prepotent over 
illegitimate pollen, when both are placed on the same stigma. 
I ascertained this by fertilising several flowers, first ille- 
gitimately, and twenty-four hours afterwards legitimately, 
with pollen taken from a peculiarly coloured variety, and 
all the seedlings were similarly coloured; this shows that 
the legitimate pollen, though applied twenty-four hours sub- 
sequently, had wholly destroyed or prevented the action of 
the previously applied illegitimate pollen. Again, as in 
making reciprocal crosses between the same two species, 
there is occasionally a great difference in the result, so the 
same thing occurs with trimorphic plants; for instance, the 
mid-styled form of Lythrum salicaria was illegitimately fer- 
tilized with the greatest ease by pollen from the longer sta- 
mens of the short-styled form, and yielded many seeds ; but 
the latter form did not yield a single seed when fertilised by 
the longer stamens of the mid-styled form. 

In all these respects, and in others which might be added, 
the forms of the same undoubted species when illegitimately 
united behave in exactly the same manner as do two distinct 
species when crossed. This led me carefully to observe 
during four years many seedlings, raised from several illegiti- 
mate unions. The chief result is that these illegitimate plants, 
as they may be called, are not fully fertile. It is possible to 
raise from dimorphic species, both long-styled and short- 
styled illegitimate plants, and from trimorphic plants all three 
illegitimate forms. These can then be properly united in a 
legitimate manner. When this is done, there is no apparent 
reason why they should not yield as many seeds as did their 
parents when legitimately fertilised. But such is not the 
case. They are all infertile, in various degrees ; some being 
so utterly and incurably sterile that they did not yield dur- 
ing four seasons a single seed or even seed-capsule. The 
sterility of these illegitimate plants, when united with each 
other in a legitimate manner, may be strictly compared with 
that of hybrids when crossed inter se. If, on the other hand, 
a hybrid is crossed with either pure parent-species, the steril- 
ity is usually much lessened; and so it is when an illegitimate 
plant is fertilised by a legitimate plant. In the same man- 


ner as the sterility of hybrids does not always run parallel 
with the difficulty of making the first cross between the two 
parent-species, so the sterility of certain illegitimate plants 
was unusually great, whilst the sterility of the union from 
which they were derived was by no means great. With hy- 
brids raised from the same seed-capsule the degree of ster- 
ility is innately variable, so it is in a marked manner with 
illegitimate plants. Lastly, many hybrids are profuse and 
persistent flowerers, whilst other and more sterile hybrids 
produce few flowers, and are weak, miserable dwarfs; 
exactly similar cases occur with the illegitimate offspring of 
various dimorphic and trimorphic plants. 

Altogether there is the closest identity in character and 
behaviour between illegitimate plants and hybrids. It is 
hardly an exaggeration to maintain that illegitimate plants are 
hybrids, produced within the limits of the same species by 
the improper union of certain forms, whilst ordinary hybrids 
are produced from an improper union between so-called dis- 
tinct species. We have also already seen that there is the 
closest similarity in all respects between first illegitimate 
unions and first crosses between distinct species. This will 
perhaps be made more fully apparent by an illustration; we 
may suppose that a botanist found two well-marked varieties 
(and such occur) of the long-styled form of the trimorphic 
Lythrum salicaria, and that he determined to try by cross- 
ing whether they were specifically distinct. He would find 
that they yielded only about one-fifth of the proper number of 
seed, and that they behaved in all the other above specified 
respects as if they had been two distinct species. But to make 
the case sure, he would raise plants from his supposed hy- 
bridized seed, and he would find that the seedlings were mis- 
erably dwarfed and utterly sterile, and that they behaved in 
all other respects like ordinary hybrids. He might then main- 
tain that he had actually proved, in accordance with the 
common view, that his two varieties were as good and as 
distinct species as any in the world; but he would be com- 
pletely mistaken. 

The facts now given on dimorphic and trimorphic plants 
are important, because they show us, first, that the physio- 
logical test of lessened fertility, both in first crosses and in 



hybrids, is no safe criterion of specific distinction; secondly, 
because we may conclude that there is some unknown bond 
which connects the infertility of illegitimate unions with 
that of their illegitimate offspring, and we are led to extend 
the same view to first crosses and hybrids; thirdly, because 
we find, and this seems to me of especial importance, that 
two or three forms of the same species may exist and may 
differ in no respect whatever, either in structure or in con- 
stitution, relatively to external conditions, and yet be sterile 
when united in certain ways. For we must remember that 
it is the union of the sexual elements of individuals of the 
same form, for instance, of two long-styled forms, which 
results in sterility; whilst it is the union of the sexual 
elements proper to two distinct forms which is fertile. Hence 
the case appears at first sight exactly the reverse of what 
occurs, in the ordinary unions of the individuals of the same 
species and with crosses between distinct species. It is, 
however, doubtful whether this is really so; but I will not 
enlarge on this obscure subject. 

We may, however, infer as probable from the consideration 
of dimorphic and trimorphic plants, that the sterility of dis- 
tinct species when crossed and of their hybrid progeny, de- 
pends exclusively on the nature of their sexual elements, and 
not on any difference in their structure or general constitu- 
tion. We are also led to this same conclusion by considering 
reciprocal crosses, in which the male of one species cannot 
be united, or can be united with great difficulty, with the 
female of a second species, whilst the converse cross can be 
effected with perfect facility. That excellent observer, Gart- 
ner, likewise concluded that species when crossed are sterile 
owing to differences confined to their reproductive systems. 


It may be urged, as an overwhelming argument, that there 
must be some essential distinction between species and vari- 
eties, inasmuch as the latter, however much they may differ 
from each other in external appearance, cross with perfect 
facility, and yield perfectly fertile offspring. With some 


exceptions, presently to be given, I fully admit that this is 
the rule. But the subject is surrounded by difficulties, for, 
looking to varieties produced under nature, if two forms 
hitherto reputed to be varieties be found in any degree sterile 
together, they are at once ranked by most naturalists as 
species. For instance, the blue and red pimpernel, which 
are considered by most botanists as varieties, are said by 
Gartner to be quite sterile when crossed, and he conse- 
quently ranks them as undoubted species. If we thus argue 
in a circle, the fertility of all varieties produced under 
nature will assuredly have to be granted. 

If we turn to varieties, produced, or supposed to have been 
produced, under domestication, we are still involved in some 
doubt. For when it is stated, for instance, that certain South 
American indigenous domestic dogs do not readily unite with 
European dogs, the explanation which will occur to every 
one, and probably the true one, is that they are descended 
from aboriginally distinct species. Nevertheless the perfect 
fertility of so many domestic races, differing widely from 
each other in appearance, for instance those of the pigeon, 
or of the cabbage, is a remarkable fact ; more especially when 
we reflect how many species there are, which, though re- 
sembling each other most closely, are utterly sterile when 
intercrossed. Several considerations, however, render the 
fertility of domestic varieties less remarkable. In the first 
place, it may be observed that the amount of external differ- 
ence between two species is no sure guide to their degree of 
mutual sterility, so that similar differences in the case of 
varieties would be no sure guide. It is certain that with 
species the cause lies exclusively in differences in their sex- 
ual constitution. Now the varying conditions to which do- 
mesticated animals and cultivated plants have been subjected, 
have had so little tendency towards modifying the repro- 
ductive system in a mr.nner leading to mutual sterility, that 
we have good grounds for admitting the directly opposite 
doctrine of Pallas, namely, that such conditions generally 
eliminate this tendency ; so that the domesticated descendants 
of species, which in their natural state probably would have 
been in some degree sterile when crossed, become perfectly 
fertile together. With plants, so far is cultivation from giving 


a tendency towards sterility between distinct species, that in 
several well-authenticated cases already alluded to, certain 
plants have been affected in an opposite manner, for they have 
become self-impotent whilst still retaining the capacity of 
fertilising, and being fertilised by, other species. If the 
Pallasian doctrine of the elimination of sterility through 
long-continued domestication be admitted, and it can hardly 
be rejected, it becomes in the highest degree improbable that 
similar conditions, long-continued should likewise induce this 
tendency; though in certain cases, with species having a 
peculiar constitution, sterility might occasionally be thus 
caused. Thus, as I believe, we can understand why with 
domesticated animals varieties have not been produced which 
are mutually sterile; and why with plants only a few such 
cases, immediately to be given, have been observed. 

The real difficulty in our present subject is not, as it ap- 
pears to me, why domestic varieties have not become mutually 
infertile when crossed, but why this has so generally occurred 
with natural varieties, as soon as they have been permanently 
modified in a sufficient degree to take rank as species. We 
are far from precisely knowing the cause ; nor is this sur- 
prising, seeing how profoundly ignorant we are in regard 
to the normal and abnormal action of the reproductive sys- 
tem. But we can see that species, owing to their struggle 
for existence with numerous competitors, will have been 
exposed during long periods of time to more uniform condi- 
tions, than have domestic varieties ; and this may well make 
a wide difference in the result. For we know how com- 
monly wild animals and plants, when taken from their natural 
conditions and subjected to captivity, are rendered sterile; 
and the reproductive functions of organic beings which have 
always lived under natural conditions would probably in like 
manner be eminently sensitive to the influence of an un- 
natural cross. Domesticated productions, on the other hand, 
which, as shown by the mere fact of their domestication, were 
not originally highly sensitive to changes in their conditions 
of life, and which can now generally resist with undiminished 
fertility repeated changes of conditions, might be expected 
to produce varieties, which would be little liable to have 
their reproductive powers injuriously affected by the act 


of crossing with other varieties which had originated in a 
like manner. 

I have as yet spoken as if the varieties of the same species 
were invariably fertile when intercrossed. But it is im- 
possible to resist the evidence of the existence of a certain 
amount of sterility in the few following cases, which I will 
briefly abstract. The evidence is at least as good as that 
from which we believe in the sterility of a multitude of spe- 
cies. The evidence is, also, derived from hostile witnesses, 
who in all other cases consider fertility and sterility as safe 
criterions of specific distinction. Gartner kept during sev- 
eral years a dwarf kind of maize with yellow seeds, and a 
tall variety with red seeds growing near each other in his 
garden ; and although these plants have separated sexes, they 
never naturally crossed. He then fertilised thirteen flowers 
of the one kind with pollen of the other; but only a single 
head produced any seed, and this one head produced only 
five grains. Manipulation in this case could not have been 
injurious, as the plants have separated sexes. No one, I 
believe, has suspected that these varieties of maize are dis- 
tinct species; and it is important to notice that the hybrid 
plants thus raised were themselves perfectly fertile; so that 
even Gartner did not venture to consider the two varieties 
as specifically different. 

Girou de Buzareingues crossed three varieties of gourd, 
which like the maize has separated sexes, and he asserts 
that their mutual fertilisation is by so much the less easy as 
their differences are greater. How far these experiments 
may be trusted, I know not; but the forms experimented 
on are ranked by Sageret, who mainly founds his classifica- 
tion by the test of infertility, as varieties, and Naudin has 
come to the same conclusion. 

The following case is far more remarkable, and seems 
at first incredible ; but it is the result of an astonishing num- 
ber of experiments made during many years on nine species 
of Verbascum, by so good an observer and so hostile a wit- 
ness as Gartner: namely that the yellow and white varieties 
when crossed produce less seed than the similarly coloured 
varieties of the same species. Moreover, he asserts that 
when yellow and white varieties of one species are crossed 


with yellow and white varieties of a distinct species, more 
seed is produced by the crosses between the similarly coloured 
flowers, than between those which are differently coloured. 
Mr. Scott also has experimented on the species and varieties 
of Verbascum; and although unable to confirm Gartner's 
results on the crossing of the distinct species, he finds that 
the dissimilarly coloured varieties of the same species yield 
fewer seeds, in the proportion of 86 to loo, than the similarly 
coloured varieties. Yet these varieties differ in no respect 
except in the colour of their flowers ; and one variety can 
sometimes be raised from the seed of another. 

Kolreuter, whose accuracy has been confirmed by every 
subsequent observer, has proved the remarkable fact, that 
one particular variety of the common tobacco was more 
fertile than the other varieties, when crossed with a widely 
distinct species. He experimented on five forms which are 
commonly reputed to be varieties, and which he tested by 
the severest trial, namely, by reciprocal crosses, and he found 
their mongrel offspring perfectly fertile. But one of these 
five varieties, when used either as the father or mother, and 
crossed with the Nicotiana glutinosa, always yielded hybrids 
not so sterile as those which were produced from the four 
other varieties when crossed with N. glutinosa. Hence the 
reproductive system of this one variety must have been 
in some manner and in some degree modified. 

From these facts it can no longer be maintained that var- 
ieties when crossed are invariably quite fertile. From the 
great difficulty of ascertaining the infertility of varieties in 
a state of nature, for a supposed variety, if proved to be in- 
fertile in any degree, would almost universally be ranked as 
a species ; — from man attending only to external characters 
in his domestic varieties, and from such varieties not hav- 
ing been exposed for very long periods to uniform conditions 
of life ; — from these several considerations we may conclude 
that fertility does not constitute a fundamental distinction 
between varieties and species when crossed. The general 
sterility of crossed species may safely be looked at, not as a 
special acquirement or endowment, but as incidental on 
changes of an unknown nature in their sexual elements. 



Independently of the question of fertility, the offspring 
of species and of varieties when crossed may be compared 
in several other respects. Gartner, whose strong wish it was 
to draw a distinct line between species and varieties, could 
find very few, and, as it seems to me, quite unimportant dif- 
ferences between the so-called hybrid offspring of species, 
and the so-called mongrel offspring of varieties. And, on the 
other hand, they agree most closely in many important re- 

I shall here discuss this subject with extreme brevity. The 
most important distinction is, that in the first generation 
mongrels are more variable than hybrids ; but Gartner admits 
that hybrids from species which have long been cultivated are 
often variable in the first generation ; and I have myself seen 
striking instances of this fact. Gartner further admits that 
hybrids between very closely allied species are more variable 
than those from very distinct species ; and this shows that 
the difference in the degree of variability graduates away. 
When mongrels and the more fertile hybrids are propagated 
for several generations, an extreme amount of variability in 
the offspring in both cases is notorious ; but some few in- 
stances of both hybrids and mongrels long retaining a uniform 
character could be given. The variability, however, in the 
successive generations of mongrels is, perhaps, greater than 
in hybrids. 

This greater variability in mongrels than in hybrids does 
not seem at all surprising. For the parents of mongrels 
are varieties, and mostly domestic varieties (very few ex- 
periments having been tried on natural varieties,) and this 
implies that there has been recent variability, which would 
often continue and would augment that arising from the act 
of crossing. The slight variability of hybrids in the first 
generation, in contrast with that in the succeeding genera- 
tions, is a curious fact and deserves attention. For it bears 
on the view which I have taken of one of the causes of 
ordinary variability; namely, that the ;-eprodiictive system 
from being emmently sensitive to changed conditions oi life, 


fails under these circumstances to perform its proper func- 
tion of producing offspring closely similar in all respects 
to the parent-form. Now hybrids in the first generation are 
descended from species (excluding those long-cultivated) 
which have not had their reproductive systems in any way 
affected, and they are not variable; but hybrids themselves 
have their reproductive systems seriously affected, and their 
descendants are highly variable. 

But to return to our comparison of mongrels and hybrids: 
Gartner states that mongrels are more liable than hybrids 
to revert to eitlier parent-form; but this, if it be true, is cer- 
tainly only a difference in degree. Moreover, Gartner ex- 
pressly states that hybrids from long cultivated plants are 
more subject to reversion than hybrids from species in their 
natural state; and this probably explains the singular differ- 
ence in the results arrived at by different observers: thus 
Max Wichura doubts whether hybrids ever revert to their 
parent-forms, and he experimented on uncultivated species 
of willows; whilst Naudin, on the other hand, insists in the 
strongest terms on the almost universal tendency to reversion 
in hybrids, and he experimented chiefly on cukivated plants. 
Gartner further states that when any two species, although 
most closely allied to each other, are crossed with a third 
species, the hybrids are widely different from each other; 
whereas if two very distinct varieties of one species are 
crossed with another species, the hybrids do not differ much. 
But this conclusion, as far as I can make out, is founded 
on a single experiment; and seems directly opposed to the 
results of several experiments made by Kolreuter. 

Such alone are the unimportant differences which Gartner 
is able to point out between hybrid and mongrel plants. On 
the other hand, the degrees and kinds of resemblance in 
mongrels and in hybrids to their respective parents, more 
especially in hybrids produced from nearly related species, 
follow according to Gartner the same laws. When two 
species are crossed, one has sometimes a prepotent power 
of impressing its likeness on the hybrid. So I believe it to 
be with varieties of plants ; and with animals one variety cer- 
tainly often has this prepotent power over another variety. 
Hybrid plants produced from a reciprocal cross, generally 


resemble each other closely ; and so it is with mongrel plants 
from a reciprocal cross. Both hybrids and mongrels can be 
reduced to either pure parent-form, by repeated crosses in 
successive generations with either parent. 

These several remarks are apparently applicable to ani- 
mals; but the subject is here much complicated, partly owing 
to the existence of secondary sexual characters; but more 
especially owing to prepotency in transmitting likeness run- 
ning more strongly in one sex than in the other, both when 
one species is crossed with another, and when one variety is 
crossed with another variety. For instance, I think those 
authors are right who maintain that the ass has a prepo- 
tent power over the horse, so that both the mule and the 
hinny resemble more closely the ass than the horse ; 
but that the prepotency runs more strongly in the male 
than in the female ass, so that the mule, which is the 
offspring of the male ass and mare, is more like an ass, 
than is the hinny, which is the offspring of the female 
ass and stallion. 

Much stress has been laid by some authors on the sup- 
posed fact, that it is only with mongrels that the offspring 
are not intermediate in character, but closely resemble one 
of their parents ; but this does sometimes occur with hy- 
brids, yet I grant much less frequently than with mongrels. 
Looking to the cases which I have collected of cross-bred 
animals closely resembling one parent, the resemblances 
seem chiefly confined to characters almost monstrous in their 
nature, and which have suddenly appeared — such as albinism, 
melanism, deficiency of tail or horns, or additional fingers 
and toes; and do not relate to characters which have been 
slowly acquired through selection. A tendency to sudden 
reversions to the perfect character of either parent would, 
also, be much more likely to occur with mongrels, which are 
descended from varieties often suddenly produced and semi- 
monstrous in character, than with hybrids, which are de- 
scended from species slowly and naturally produced. On the 
whole, I entirely agree with Dr. Prosper Lucas, who. after 
arranging an enormous body of facts with respect to animals, 
comes to the conclusion that the laws of resemblance of the 
child to its parents are the same, whether the two parents 


differ little or much from each other, namely, in the union 
of individuals of the same variety, or of different varieties, 
or of distinct species. 

Independently of the question of fertility and sterility, in 
all other respects there seems to be a general and close simi- 
larity in the offspring of crossed species, and of crossed vari- 
eties. If we look at species as having been specially created, 
and at varieties as having been produced by secondary laws, 
this similarity would be an astonishing fact. But it har- 
monises perfectly with the view that there is no essential 
distinction between species and varieties. 


First crosses between forms, sufficiently distinct to be 
ranked as species, and their hybrids, are very generally, 
but not universally, sterile. The sterility is of all degrees, 
and is often so slight that the most careful experimentalists 
have arrived at diametrically opposite conclusions in ranking 
forms by this test. The sterility is innately variable in indi- 
viduals of the same species, and is eminently susceptible to 
the action of favourable and unfavourable conditions. The 
degree of sterility does not strictly follow systematic affinity, 
but is governed by several curious and complex laws. It is 
generally different, and sometimes widely different in 
reciprocal crosses between the same two species. It is not 
always equal in degree in a first cross and in the hybrids 
produced from this cross. 

In the same manner as in grafting trees, the capacity in 
one species or variety to take on another, is incidental on 
differences, generally of an unknown nature, in their vege- 
tative systems, so in crossing, the greater or less facility of 
one species to unite with another is incidental on unknown 
differences in their reproductive systems. There is no more 
reason to think that species have been specially endowed 
with various degrees of sterility to prevent their crossing 
and blending in nature, than to think that trees have been 
specially endowed with various and somewhat analogous 
degrees of difficulty in being grafted together in order to pre- 
vent their inarching in our forests. 


The sterility of first crosses and of their hybrid progeny 
has not been acquired through natural selection. In the 
case of first crosses it seems to depend on several circum- 
stances; in some instances in chief part on the early death 
of the embryo. In the case of hybrids, it apparently depends 
on their whole organisation having been disturbed by being 
compounded from two distinct forms; the sterility being 
closely allied to that which so frequently affects pure species, 
when exposed to new and unnatural conditions of life. He 
who will explain these latter cases will be able to explain 
the sterility of hybrids. This view is strongly supported by 
a parallelism of another kind: namely, that, firstly, slight 
changes in the conditions of life add to the vigour and fertil- 
ity of all organic beings ; and secondly, that the crossing of 
forms, which have been exposed to slightly different condi- 
tions of life or which have varied, favours the size, vigour, 
and fertility of their offspring. The facts given on the 
sterility of the illegitimate unions of dimorphic and trimor- 
phic plants and of their illegitimate progeny, perhaps ren- 
der it probable that some unknown bond in all cases connects 
the degree of fertility of first unions with that of their 
offspring. The consideration of these facts on dimorphism, 
as well as of the results of reciprocal crosses, clearly leads 
to the conclusion that the primary cause of the sterility 
of crossed species is confined to differences in their sexual 
elements. But why, in the case of distinct species, the sexual 
elements should so generally have become more or less modi- 
fied, leading to their mutual infertility, we do not know; 
but it seems to stand in some close relation to species hav- 
ing been exposed for long periods of time to nearly uniform 
conditions of life. 

It is not surprising that the difficulty in crossing any two 
species, and the sterility of their hybrid offspring, should 
in most cases correspond, even if due to distinct causes: for 
both depend on the amount of difference between the species 
which are crossed. Nor is it surprising that the facility of 
effecting a first cross, and the fertility of the hybrids thus 
produced, and the capacity of being grafted together — though 
this latter capacity evidently depends on widely different cir- 
cumstances — should all run, to a certain extent, parallel with 


the systematic affinity of the forms subjected to experiment; 
for systematic affinity includes resemblances of all kinds. 

First crosses between forms known to be varieties, or suffi- 
ciently alike to be considered as varieties, and their mon- 
grel offspring, are very generally, but not, as is so often 
stated, invariably fertile. Nor is this almost universal and 
perfect fertility surprising, when it is remembered how 
liable we are to argue in a circle with respect to varieties 
in a state of nature ; and when we remember that the greater 
number of varieties have been produced under domestication 
by the selection of mere external differences, and that they 
have not been long exposed to uniform conditions of life. It 
should also be especially kept in mind, that long-continued 
domestication tends to eliminate sterility, and is therefore 
little likely to induce this same quality. Independently of the 
question of fertility, in all other respects there is the closest 
general resemblance between hybrids and mongrels, — in their 
variability, in their power of absorbing each other by re- 
peated crosses, and in their inheritance of characters from 
both parent-forms. Finally, then, although we are as ig- 
norant of the precise cause of the sterility of first crosses 
and of hybrids as we are why animals and plants removed 
from their natural conditions become sterile, yet the facts 
given in this chapter do not seem to me opposed to the belief 
that species aboriginally existed as varieties. 

On the Imperfection of the Geological Record 

On the absence of intermediate varieties at the present day — On the 

nature of extinct intermediate varieties ; on their number — On 
the lapse of time, as inferred from the rate of denudation and 
of deposition — On the lapse of time as estimated by years — 
On the poorness of our palseontological collections — On the in- 
termittence of geological formations — On the denudation of 
granitic areas — On the absence of intermediate varieties in any 
one formation — On the sudden appearance of groups of species 
— On their sudden appearance in the lowest known fossiliferous 
strata — Antiquity of the habitable earth. 

IN the sixth chapter I enumerated the chief objections 
which might be justly urged against the views main- 
tained in this vohime. Most of them have now been dis- 
cussed. One, namely the distinctness of specific forms, and 
their not being blended together by innumerable transitional 
links, is a very obvious difficulty. I assigned reasons why 
such links do not commonly occur at the present day under 
the circumstances apparently most favourable for their pres- 
ence, namely on an extensive and continuous area with grad- 
uated physical conditions. I endeavoured to show, that the 
life of each species depends in a more important manner on 
the presence of other already defined organic forms, than on 
climate, and, therefore, that the really governing conditions 
of life do not graduate away quite insensibly like heat or 
moisture. I endeavoured, also, to show that intermediate va- 
rieties, from existing in lesser numbers than the forms which 
they connect, will generally be beaten out and exterminated 
during the course of further modification and improvement. 
The inain cause, however, of innumerable intermediate links " 
not now occurring everywhere throughout nature, depends on 
the very process of natural selection, through which new va- 
rieties continually take the places of and supplant their 
parent-forms. But just in proportion as this process of ex- 



termination has acted on an enormous scale, so must the 
number of intermediate varieties, which have formerly ex- 
isted, be truly enormous. Why then is not every geological 
^formation and every stratum full of such intermediate links? 
Geology assuredly does not reveal any such finely-graduated 
organic chain; and this, perhaps, is the most obvious and 
serious objection which can be urged against the theory. The 
explanation lies, as I believe, in the extreme imperfection of 
the geological record. 

In the first place, it should always be borne in mind what 
sort of intermediate forms must, on the theory, have formerly 
existed. I have found it difficult, when loolcing at any two 
species, to avoid picturing to myself forms directly intermedi- 
ate between them. But this is a wholly false view ; we should 
always look for forms intermediate between each species and 
a common but unknown progenitor; and the progenitor will 
generally have differed in some respects from all its modified 
descendants. To give a simple illustration: the fantail and 
pouter pigeons are both descended from the rock-pigeon; if 
we possessed all the intermediate varieties which have ever 
existed, we should have an extremely close series between 
both and the rock-pigeon ; but we should have no varieties 
directly intermediate between the fantail and pouter; none, 
for instance, combining a tail somewhat expanded with a crop 
somewhat enlarged, the characteristic features of these two 
breeds. These two breeds, moreover, have become so much 
modified, that, if we had no historical or indirect evidence 
regarding their origin, it would not have been possible to 
have determined, from a mere comparison of their structure 
with that of the rock-pigeon, C. Hvia, whether they had de- 
scended from this species or from some other allied form, 
such as C. oenas. 

So, with natural species, if we look to forms very distinct, 
for instance to the horse and tapir, we have no reason to 
suppose that links directly intermediate between them ever 
existed, but between each and an unknown common parent. 
The common parent will have had in its whole organisation 
much general resemblance to the tapir and to the horse; but 
in some points of structure may have differed considerably 
from both, even perhaps more than they differ from each 


other. Hence, in all such cases, we should be unable to rec- 
ognise the parent-form of any two or more species, even if 
we closely compared the structure of the parent with that of 
its modified descendants, unless at the same time we had a 
nearly perfect chain of the intermediate links. 

It is just possible by the theory, that one of two living 
forms might have descended from the other; for instance, a 
horse from a tapir; and in this case direct intermediate links 
will have existed between them. But such a case would im- 
ply that one form had remained for a very long period unal- 
tered, whilst its descendants had undergone a vast amount 
of change ; and the principle of competition between organism 
and organism, between child and parent, will render this a 
very rare event ; for in all cases the new and improved forms 
of life tend to supplant the old and unimproved forms. 

By the theory of natural selection all living species have 
been connected with the parent-species of each genus, by dif- 
ferences not greater than we see between the natural and 
domestic varieties of the same species at the present day ; and 
these parent-species, now generally extinct, have in their 
turn been similarly connected with more ancient forms ; and 
so on backwards, always converging to the common ancestor 
of each great class. So that the number of intermediate and 
transitional links, between all living and extinct species, must 
have been inconceivably great. But assuredly, if this theory 
be true, such have lived upon the earth. 


Independently of our not finding fossil remains of such in- 
finitely numerous connecting links, it may be objected that 
time cannot have sufficed for so great an amount of organic 
change, all changes having been effected slowly. It is hardly 
possible for me to recall to the reader who is not a practical 
geologist, the facts leading the mind feebly to comprehend the 
lapse of time. He who can read Sir Charles Lyell's grand 
work on the Principles of Geology, which the future historian 
will recognise as having produced a revolution in natural 
science, and yet does not admit how vast have been the past 


periods of time, may at once close this volume. Not that it 
suffices to study the Principles of Geology, or to read special 
treatises by different observers on separate formations, and 
to mark how each author attempts to give an inadequate idea 
of the duration of each formation, or even of each stratum. 
We can best gain some idea of past time by knowing the 
agencies at work, and learning how deeply the surface of the 
land has been denuded, and how much sediment has been de- 
posited. As Lyell has well remarked, the extent and thick- 
ness of our sedimentary formations are the result and the 
measure of the denudation which the earth's crust has else- 
where undergone. Therefore a man should examine for 
himself the great piles of superimposed strata, and watch the 
rivulets bringing down mud. and the waves wearing away the 
sea-cliffs, in order to comprehend something about the dura- 
tion of past time, the monuments of which we see all 
around us. 

It is good to wander along the coast, when formed of mod- 
erately hard rocks, and mark the process of degradation. The 
tides in most cases reach the cliffs only for a short time twice 
a day, and the waves eat into them only when they are 
charged with sand or pebbles ; for there is good evidence that 
pure water effects nothing in wearing away rock. At last 
the base of the cliff is undermined, huge fragments fall down, 
and these, remaining fixed, have to be worn away atom by 
atom, until after being reduced in size they can be rolled 
about by the waves, and then they are more quickly ground 
into pebbles, sand, or mud. But how often do we see along 
the bases of retreating cliffs rounded boulders, all thickly 
clothed by marine productions, showing how little they are 
abraded and how seldom they are rolled about ! Moreover, 
if we follow for a few miles any line of rocky cliff, which is 
undergoing degradation, we find that it is only here and there, 
along a short length or round a promontory, that the cliffs 
are at the present time suffering. The appearance of the sur- 
face and the vegetation show that elsewhere years have 
elapsed since the waters washed their base. 

We have, however, recently learnt from the observations 
of Ramsay, in the van of many excellent observers — of Jukes, 
Geikie, Croll, and others, that subaerial degradation is a 


much more important agency than coast-action, or the power 
of the waves. The whole surface of the land is exposed to 
the chemical action of the air and of the rain-water with its 
dissolved carbonic acid, and in colder countries to frost; the 
disintegrated matter is carried down even gentle slopes dur- 
ing heavy rain, and to a greater extent than might be sup- 
posed, especially in arid districts, by the wind ; it is then 
transported by the streams and rivers, which when rapid 
deepen their channels, and triturate the fragments. On a 
rainy day, even in a gently undulating country, we see the 
effects of subaerial degradation in the muddy rills which flow 
down every slope. Messrs. Ramsay and Whitaker have 
shown, and the observation is a most striking one, that the 
great lines of escarpment in the Wealden district and those 
ranging across England, which formerly were looked at as 
ancient sea-coasts, cannot have been thus formed, for each 
line is composed of one and the same formation, whilst our 
sea-cliffs are everywhere formed by the intersection of vari- 
ous formations. This being the case, we are compelled to 
admit that the escarpments owe their origin in chief part to 
the rocks of which they are composed having resisted subae- 
rial denudation better than the surrounding surface ; this sur- 
face consequently has been gradually lowered, with the lines 
of harder rock left projecting. Nothing impresses the mind 
with the vast duration of time, according to our ideas of time, 
more forcibly than the conviction thus gained that subaerial 
agencies which apparently have so little power, and which 
seem to work so slowly, have produced great results. 

When thus impressed with the slow rate at which the land 
is worn away through subaerial and littoral action, it is good, 
in order to appreciate the past duration of time, to consider 
on the one hand, the masses of rock which have been re- 
moved over many extensive areas, and on the other hand the 
thickness of our sedimentary formations. I remember hav- 
ing been much struck when viewing volcanic islands, which 
have been worn by the waves and pared all round into per- 
pendicular cVius of one or two thousand feet in height ; for 
the gentle slope of the .lava-streams, due to their formerly 
liquid state, showed at a glance how far the hard, rocky beds 
had once extended into the open ocean. The same story is 


told still more plainly by faults, — those great cracks along 
which the strata have been upheaved on one side, or thrown 
down on the other, to the height or depth of thousands of 
feet; for since the crust cracked, and it makes no great dif- 
ference whether the upheaval was sudden, or, as most geolo- 
gists now believe, was slow and effected by many starts, the 
surface of the land has been so completely planed down that 
no trace of these vast dislocations is externally visible. The 
Craven fault, for instance, extends for upwards of 30 miles, 
and along this line the vertical displacement of the strata 
varies from 600 to 3000 feet. Professor Ramsay has pub- 
lished an account of a downthrow in Anglesea of 2300 feet; 
and he informs me that he fully believes that there is one in 
Merionethshire of 12,000 feet; yet in these cases there 19 
nothing on the surface of the land to show such prodigious 
movements; the pile of rocks on either side of the crack 
having been smoothly swept away. 

On the other hand, in all parts of the world the piles of 
sedimentary strata are of wonderful thickness. In the Cor- 
dillera I estimated one mass of conglomerate at ten thou- 
sand feet; and although conglomerates have probably been 
accumulated at a quicker rate than finer sediments, yet from 
being formed of worn and rounded pebbles, each of which 
bears the stamp of time, they are good to show how slowly 
the mass must have been heaped together. Professor Ramsay 
has given me the maximum thickness, from actual measure- 
ment in most cases, of the successive formations in different 
parts of Great Britain; and this is the result: — 


Palaeozoic strata (not including igneous beds) 57.154 

Secondary strata *3.i90 

Tertiary strata 2,240 

— making altogether 72,584 feet; that is, very nearly thirteen 
and three-quarters British miles. Some of the formations, 
which are represented in England by thin beds, are thousands 
of feet in thickness on the Continent. Moreover, between 
each successive formation, we have, in the opinion of most 
geologists, blank periods of enormous length. So that the 
lofty pile of sedimentary rocks in Britain gives but an inade- 
quate idea of the time which has elapsed during their accu- 


mulation. The consideration of these various facts impresses 
the mind almost in the same manner as does the vain en- 
deavour to grapple with the idea of eternity. 

Nevertheless this impression is partly false. Mr. Croll, in 
an interesting paper, remarks that we do not err ''in forming 
too great a conception of the length of geological periods," 
but in estimating them by years. When geologists look at 
large and complicated phenomena, and then at the figures rep- 
resenting several million years, the two produce a totally 
different effect on the mind, and the figures are at once pro- 
nounced too small. In regard to subaerial denudation, Mr. 
Croll shows, by calculating the known amount of sediment 
annually brought down by certain rivers, relatively to their 
areas of drainage, that looo feet of solid rock, as it became 
gradually disintegrated, would thus be removed from the 
mean level of the whole area in the course of six million 

This seems an astonishing result, and some considera- 
tions lead to the suspicion that it may be too large, but even 
if halved or quartered it is still very surprising. Few of us, 
however, know what a million really means : Mr. Croll gives 
the following illustration: take a narrow strip of paper, 83 
feet 4 inches in length, and stretch it along the wall of a large 
hall; then mark off at one end the tenth of an inch. This 
tenth of an inch will represent one hundred years, and the 
entire strip a million years. But let it be borne in mind, in 
relation to the subject of this work, what a hundred years 
implies, represented as it is by a measure utterly insignificant 
in a hall of the above dimensions. Several eminent breeders, 
during a single lifetime, have so largely modified some of the 
higher animals, which propagate their kind much more slowly 
than most of the lower animals, that they have formed what 
well deserves to be called a new sub-breed. Few men have 
attended with due care to any one strain for more than half 
a century, so that a hundred years represents the work of two 
breeders in succession. It is not to be supposed that species 
in a state of nature ever change so quickly as domestic ani- 
mals under the guidance of methodical selection. The com- 
parison would be in every way fairer with the effects which 
follow from unconscious selection, that is the preservation of 


the most useful or beautiful animals, with no intention of 
modifying the breed; but by this process of unconscious 
selection, various breeds have been sensibly changed in the 
course of two or three centuries. 

Species, however, probably change much more slowly, and 
within the same country only a few change at the same time. 
This slowness follows from all the inhabitants of the same 
country being already so well adapted to each other, that new 
places in the polity of nature do not occur until after long 
intervals, due to the occurrence of physical changes of some 
kind, or through the immigration of new forms. Moreover 
variations or individual differences of the right nature, by 
which some of the inhabitants might be better fitted to their 
new places under the altered circumstances, would not always 
occur at once. Unfortunately we have no means of deter- 
mining, according to the standard of years, how long a 
period it takes to modify a species; but to the subject of time 
we must return. 


Now let us turn to our richest geological museums, and 
what a paltry display we behold ! That our collections are 
imperfect is admitted by every one. The remark of that ad- 
mirable palaeontologist, Edward Forbes, should never be for- 
gotten, namely, that very many fossil species are known and 
named from single and often broken specimens, or from a 
few specimens collected on some one spot. Only a small por- 
"1^ tion of the surface of the earth has been geologically ex- 
plored, and no part with sufficient care, as the important 
discoveries made every year in Europe prove. No organism 
^ wholly soft can be preserved. Shells and bones decay and 
disappear when left on the bottom of the sea, where sediment 
is not accumulating. We probably take a quite erroneous 
view, when we assume that sediment is being deposited over 
nearly the whole bed of the sea, at a rate sufficiently quick 
to embed and preserve fossil remains. Throughout an enor- 
mously large proportion of the ocean, the bright blue tint of 
the water bespeaks its purity. The many cases on record of 
a formation conformably covered, after an immense interval 


of time, by another and later formation, without the under- 
lying bed having suffered in the interval any wear and tear, 
seem explicable only on the view of the bottom of the sea not 
rarely lying for ages in an unaltered condition. The remains^ 
which do become embedded, if in sand or gravel, will, when 
the beds are upraised, generally be dissolved by the percola- 
tion of rain-water charged with carbonic acid. Some of the 
many kinds of animals which live on the beach between high '' 
and low water mark seem to be rarely preserved. For in- 
stance, the several species of the Chthamalinae (a sub-family 
of sessile cirripedes) coat the rocks all over the world in 
infinite numbers ; they are all strictly littoral, with the excep- 
tion of a single Mediterranean species, which inhabits deep 
water, and this has been found fossil in Sicily, whereas not 
one other species has hitherto been found in any tertiary 
formation ; yet it is known that the genus Chthamalus ex- 
isted during the Chalk period. Lastly, many great deposits 
requiring a vast length of time for their accumulation, are 
entirely destitute of organic remains, without our being able 
to assign any reason: one of the most striking instances is 
that of the Flysch formation, which consists of shale and 
sandstone, several thousand, occasionally even six thousand 
feet in thickness, and extending for at least 300 miles from 
Vienna to Switzerland; and although this great mass has 
been most carefully searched, no fossils, except a few vege- 
table remains, have been found. 

With respect to the terrestrial productions which lived 
during the Secondary and Palaeozoic periods, it is superfluous 
to state that our evidence is fragmentary in an extreme de- 
gree. For instance, until recently not a land-shell was known 
belonging to either of these vast periods, with the exception 
of one species discovered by Sir C. Lycll and Dr. Dawson in 
the carboniferous strata of North America ; but now land- 
shells have been found in the lias. In regard to mammifer-'~^ 
ous remains, a glance at the historical table published in 
Lyell's Manual wmII bring home the truth, how accidental and 
rare is their preservation, far better than pages of detail. 
Nor is their rarity surprising, when we remember how large ; 
a proportion of the bones of tertiary mammals have been 
discovered either in caves or in lacustrine deposits ; and that 


not a cave or true lacustrine bed is known belonging to the 
age of our secondary or palaeozoic formations. 
y^ But the imperfection in the geological record largely re- 
sults from another and more important cause than any of the 
foregoing; namely, from the several formations being sep- 
arated from each other by wide intervals of time. This doc- 
trine has been emphatically admitted by many geologists and 
palaeontologists, who, like E. Forbes, entirely disbelieve in 
the change of species. When we see the formations tabulated 
in written works, or when we follow them in nature, it is 
difficult to avoid believing that they are closely consecutive. 
But we know, for instance, from Sir R. Murchison's great 
work on Russia, what wide gaps there are in that country 
between the superimposed formations ; so it is in North 
America, and in many other parts of the world. The most 
skilful geologist, if his attention had been confined exclusively 
to these large territories, would never have suspected that, 
during the periods which were blank and barren in his own 
country, great piles of sediment, charged with new and pe- 
culiar forms of life, had elsewhere been accumulated. And 
if, in each separate territory, hardly any idea can be formed 
of the length of time which has elapsed between the consecu- 
tive formations, we may infer that this could nowhere be 
ascertained. The frequent and great changes in the mineral- 
ogical composition of consecutive formations, generally im- 
plying great changes in the geography of the surrounding 
lands, whence the sediment was derived, accord with the 
belief of vast intervals of time having elapsed between each 
C' We can, I think, see why the geological formations of each 
\ region are almost invariably intermittent; that is, have not 
1 followed each other in close sequence. Scarcely any fact 
struck me more when examining many hundred miles of the 
South American coasts, which have been upraised several 
hundred feet within the recent period, than the absence of 
any recent deposits sufficiently extensive to last for even a 
short geological period. Along the whole west coast, which 
is inhabited by a peculiar marine fauna, tertiary beds are so 
poorly developed, that no record of several successive and 
peculiar marine faunas will probably be preserved to a distant 


age. A little reflection will explain why, along the rising 
coast of the western side of South Ameiica, no extensive 
formations with recent or tertiary remains can anywhere be 
found, though the supply of sediment must for ages havQ 
been great, from the enormous degradation of the coast-rocks 
and from muddy streams entering the sea. The explanation,' 
no doubt, is, that the littoral and sub-littoral deposits are 
continually worn away, as soon as they are brought up by 
the slow and gradual rising of the land within the grinding 
action of the coast-waves. 

We may, I think, conclude tha t sediment must be accumu- 
lat ed in ext remely thick, soird^or extensive m asses, in order 
to withstand the incessant act ion "oF'the waves, when first 
upraised and during successive oscillations of level, as well as 
the subsequent subaerial degradation. Such thick and ex- 
tensTve^'"Sx:"cumulations of sediment may be formed in two ' 
ways ; either in profound depths of the sea, in which case~~] 
the bottom will not be inhabited by so many and such varied _J 
forms of life, as the more shallow seas; and the mass when 
upraised will give an imperfect record of the organisms 
which existed in the neighbourhood during the period of its 
accumulation. Or, sediment may be deposited to any thick- 
ness and extent over a shallow bottom, if it continue slowly 
to subside. In this latter case, as long as the rate of subsi- 
dence and the supply of sediment nearly balance each other, ^ 
the sea will remain shallow and favourable for many and 
varied forms, and thus a rich fossiliferous formation, thick 
enough, when upraised, to resist a large amount of denuda- 
tion, may be formed. 

I am convinced that nearly all our ancient formations, 
which are throughout the greater part of their thickness rich 
in fossils, have thus been formed during subsidence. Since 
publishing my views on this subject in 1845, I have watched 
the progress of Geology, and have been surprised to note how 
author after author, in treating of this or that great forma- 
tion, has come to the conclusion that it was accumulated 
during subsidence. I may add, that the only ancient tertiary 
formation on the west coast of South America, which has 
been bulky enough to resist such degradation as it has as yet 
suffered, but which will hardly last to a distant geological 


age, was deposited during a c?.*:i.-ird cscfZsr:!?:: of 
and ditts gained ctmsiderakie thirkness. 

An geologica] facts tell ns plaiohr that each 3.rt3. -3^ under- 
gone nameroas 5lo"w osciilations of level, and ^.r^Mrentlv these 
oscillaticHis bave asected wide spaces. Coaisectienthr, foraaft- 
tions rich in fossils and sofBaently thick and extensile to 
resist snhsequient degradaticm, will have been :£cKined otO" 
wule spaces dnring periods ci subsidence, bet only wirere tiie 
supply of sediment was sufficient to keep the sea sbaDow and 
to embed and preserve the resnains before tfaey had time to 
decay. On die other hand, as l(Xig as the bed of the sea 
remains stationary, tkick deports cannot have been accnoni- 
tated in the shallow parts, which are the most favoarafaie to 
Hfe. Still less can this have ha£^>ened cfaning the ahemate 
periods of elevation: or, to speak more accnratdy, tfse be<^ 
which "w^re then acctmmlated will geaerally have beoi de- 
stroved bv being C5)raised and toongfat within the famits of 
the coast-action. 

These remarks apply chieSy to Uttoral and snb-Iittoral de- 
posits. In the case of an extensive and ^bDow sea, snch as 
that within a large part of the Malay .\rchipelago, where the 
depdi varies from 30 or 40 to 60 fathcHns, a wid>dT extended 
formation might be formed dnring a period of deration, and 
yet not suffer excessively from denodation daring its slow 
iq^ieaval; bat the thickness of the formation could not be 
great, fcr owing to the elevarory movemsit it wookl be less 
than the depth in which it was formed: nor would the deposit 
be much consolidated, nor be capped by overlying formatiaasy 
so that it wcnli rm a good chance of being worn away by 
atmospheric degradation and by the action of the sea during 
subsequent oscillaticns of leveL It has. however, been sug- 
gested by Mr. Hopkins, that if one part of the area, after 
rising and before being denuded. subsMied. the deposit formed 
dtrrins^ the rising movement, thoogfa not thick, might after- 
wards become protectef '" " ""esh accumulatioas. and thus be 
preserved for a long per 

Mr. Hopkins also expresses his belief that sedmtentarj beds 
of considerable horizontal extent have r^- ' tij 

destroyed. Bet all geologists, excepting _; :. "^ 

that our present metamorphic schists and pltttomc r . -.- ■ :ice 


formed the primordial nucleus of the globe, will admit that 
these latter rocks have been stript of their covering to an 
enormous extent. For it is scarcely possible that such rocks 
could have been solidified and crystallized whilst uncovered; 
but if the metamorphic action occurred at profound depths of 
the ocean, the former protecting mantle of rock may not have 
been ver>- thick. Admitting then that gneiss, mica-schist, 
granite, diorite, &c., were once necessarily covered up, how 
can we account for the naked and extensive areas of such 
rocks in many parts of the world, except on the belief that 
they have subsequently been completely denuded of all over- 
lying strata? That such extensive areas do exist cannot be 
doubted; the granitic region of Parime is described by Hum- 
boldt as being at least nineteen times as large as Switzerland. 
South of the Amazon, Boue colours an area composed of 
rocks of this nature as equal to that of Spain, France, Italy, 
part of Germany, and the British Islands, all conjoined. This 
region has not been carefully explored, but from the concur- 
rent testimony of travellers, the granitic area is very large; 
thus, Von Eschwege gives a detailed section of these rocks, 
stretching from Rio de Janeiro for 260 geographical miles 
inland in a straight line; and I travelled for 150 miles in 
another direction, and saw nothing but granitic rocks. Nu- 
merous specimens, collected along the whole coast from near 
Rio Janeiro to the mouth of the Plata, a distance of iioo geo- 
graphical miles, were examined by me, and they all belonged 
to this class. Inland, along the whole northern bank of the 
Plata I saw, besides modern tertiary beds, only one small 
patch of slightly metamorphosed rock, which alone could 
have formed a part of the original capping of the granitic 
series. Turning to a well-known region, namely, to the 
United States and Canada, as shown in Professor H. D. 
Rogers's beautiful map, I have estimated the areas by cutting 
out and weighing the paper, and I find that the metamorphic 
(excluding "the semi-metamorphic") and granitic rocks ex- 
ceed, in the proportion of 19 to 12-5, the whole of the newer 
Palaeozoic formations. In many regions the metamorphic and 
granitic rocks would be found much more widely extended 
than they appear to be, if all the sedimentary beds were re- 
moved which rest unconformably on them, and which could 


not have formed part of the original mantle under which they 
were crystallized. Hence it is probable that in some parts 
of the world whole formations have been completely de- 
nuded, with not a wreck left behind. 

One remark is here worth a passing notice. During periods 
of elevation the area of the land and of the adjoining shoal 
parts of the sea will be increased, and new stations will often 
be formed : — all circumstances favourable, as previously ex- 
plained, for the formation of new varieties and species; but 
during such periods there will generally be a blank in the 
geological record. On the other hand, during subsidence, the 
inhabited area and number of inhabitants will decrease (ex- 
cepting on the shores of a continent when first broken up into 
an archipelago), and consequently during subsidence, though 
there will be much extinction, few new varieties or species 
will be formed; and it is during these very periods of subsi- 
dence, that the deposits which are richest in fossils have been 


/ From these several considerations, it cannot be doubted 
i that the geological record, viewed as a whole, is extremely 
I imperfect; but if we confine our attention to any one forma- 
tion, it becomes much more difficult to understand why we do 
not therein find closely graduated varieties between the allied 
species which lived at its commencement and at its close. 
Several cases are on record of the same species presenting 
varieties in the upper and lower parts of the same formation ; 
thus, Trautschold gives a number of instances with Ammo- 
nites ; and Hilgendorf has described a most curious case of 
ten graduated forms of Planorbis multiformis in the succes- 
sive beds of a fresh-water formation in Switzerland. Although 
each formation has indisputably required a vast number of 
years for its deposition, several reasons can be given why 
each should not commonly include a graduated series of links 
between the species which lived at its commencement and 
close; but I cannot assign due proportional weight to the 
following considerations. 


Although each formation may mark a very long lapse of 
years, each probably is short compared with the period requi 
site to change one species into another. I am aware that two 
palaeontologists, whose opinions are worthy of much defer- 
ence, namely Bronn and Woodward, have concluded that the 
average duration of each formation is twice or thrice as long 
as the average duration of specific forms. But insuperable 
difficulties, as it seems to me, prevent us from coming to any 
just conclusion on this head. When we see a species first 
appearing in the middle of any formation, it would be rash 
in the extreme to infer that it had not elsewhere previously 
existed. So again when we find a species disappearing before 
the last layers have been deposited, it would be equally rash 
to suppose that it then became extinct. We forget how small 
the area of Europe is compared with the rest of the world; 
nor have the several stages of the same formation throughout 
Europe been correlated with perfect accuracy. / 

We may safely infer that with marine animals of all kinds] ^ 
there has been a large amount of migration due to climatalj ^^j^S 
and other changes; and when we see a species first appearing\^ 
in any formation, the probability is that it only then first im- 
migrated into that area. It is well known, for instance, that 
several species appear somewhat earlier in the palaeozoic beds 
of North America than in those of Europe ; time having ap- 
parently been required for their migration from the American 
to the European seas. In examining the latest deposits in 
various quarters of the world, it has everywhere been noted, 
that some few still existing species are common in the de- 
posit, but have become extinct in the immediately surround- 
ing sea ; or, conversely, that some are now abundant in the 
neighbouring sea, but are rare or absent in this particular 
deposit. It is an excellent lesson to reflect on the ascer^ v 
tained amount of migration of the inhabitants of Europe dur- I *^ 
ing the glacial epoch, which forms only a part of one whole J "^ 
geological period; and likewise to reflect on the changes of 4 ^* 
level, on the extreme change of climate, and on the great "1^^ *' 
lapse of time, all included within this same glacial period. I .v--^ 
Yet it may be doubted whether, in any quarter of the world, 
sedimentary deposits, including fossil remains, have gone on 
accumulating within the same area during the whole of this 


period. It is not, for instance, probable that sediment was 
deposited during the whole of the glacial period near the 
mouth of the Mississippi, within that limit of depth at which 
marine animals can best flourish: for we know that great 
geographical changes occurred in other parts of America dur- 
ing this space of time. When such beds as were deposited in 
shallow water near the mouth of the Mississippi during some 
part of the glacial period shall have been upraised, organic 
remains will probably first appear and disappear at different 
levels, owing to the migrations of species and to geographical 
changes. And in the distant future, a geologist, examining 
those beds, would be tempted to conclude that the average 
duration of life of the embedded fossils had been less than 
that of the glacial period, instead of having been really far 
greater, that is, extending from before the glacial epoch to 
the present day. 

In order to get a perfect gradation between two forms in 
the upper and lower parts of the same formation, the deposit 
must have gone on continuously accumulating during a long 
period, sufficient for the slow process of modification; hence 
the deposit must be a very thick one ; and the species under- 
going change must have lived in the same district throughout 
the whole time. But we have seen that a thick formation, 
fossiliferous throughout its entire thickness, can accumulate 
only during a period of subsidence ; and to keep the depth ap- 
proxi'mately the same, which is necessary that the same 
marine species may live on the same space, the supply of 
sediment must nearly counterbalance the amount of subsi- 
dence. But this same movement of subsidence will tend to 
submerge the area whence the sediment is derived, and thus 
diminish the supply, whilst the downward movement con- 
tinues. In fact, this nearly exact balancing between the 
supply of sediment and the amount of subsidence is probably 
a rare contingency; for it has been observed by more than 
one palaeontologist, that very thick deposits are usually 
barren of organic remains, except near their upper or lower 

It would seem that each separate formation, like the whole 
pile of formations in any country, has generally been inter- 
mittent in its accumulation. When we see, as is so often the 


case, a formation composed of beds of widely different min- 
eralogical composition, we may reasonably suspect that the 
process of deposition has been more or less interrupted. Nor 
will the closest inspection of a formation give us any idea of 
the length of time which its deposition may have consumed. 
Many instances could be given of beds only a few feet 
in thickness, representing formations, which are elsewhere 
thousands of feet in thickness, and which must have required 
an enormous period for their accumulation ; yet no one igno- 
rant of this fact would have even suspected the vast lapse of 
time represented by the thinner formation. Many cases could 
be given of the lower beds of a formation having been up- 
raised, denuded, submerged, and then re-covered by the upper 
beds of the same formation, — facts, showing what wide, yet 
easily overlooked, intervals have occurred in its accumula- 
tion. In other cases we have the plainest evidence in great 
fossilised trees, still standing upright as they grew, of many 
long intervals of time and changes of level during the process 
of deposition, which would not have been suspected, had not 
the trees been preserved : thus Sir C. Lyell and Dr. Dawson 
found carboniferous beds 1400 feet thick in Nova Scotia, with 
ancient root-bearing strata, one above the other at no less 
than sixty-eight different levels. Hence, when the same 
species occurs at the bottom, middle, and top of a formation, 
the probability is that it has not lived on the same spot during 
the whole period of deposition, but has disappeared and reap- 
peared, perhaps many times, during the same geological 
period. Consequently if it were to undergo a considerable 
amount of modification during the deposition of any one geo- 
logical formation, a section would not include all the fine 
interme diate gradatTotis~which must ofi our IHeotr ijl^ve ex- 
iste d.'but abrupt, tho ugh perhaps slight, changes of form. 

it is all-important to remember that naturalists have no 
golden rule by which to distinguish species and varieties; 
they grant some little variability to each species, but when 
they meet with a somewhat greater amount of difference be- 
tween any two forms, they rank both as species, unless they 
are enabled to connect them together by the closest inter- 
mediate gradations; and this, from the reasons just assigned, 
we can seldom hope to effect in any one geological section. 



Supposing B and C to be two species, and a third, A, to be 
found in an older and underlying bed ; even if A were strictly 
intermediate between B and C it would simply be ranked as a 
third and distinct species, unless at the same time it could be 
closely connected by intermediate varieties with either one or 
both forms. Nor should it be forgotten, as before explained, 
that A might be the actual progenitor of B and C, and yet 
would not necessarily be strictly intermediate between them 
r in all respects. So that we might obtain the parent-species 
and its several modified descendants from the lower and 
upper beds of the same formation, and unless we obtained 
numerous transitional gradations, we should not recognise 
their blood-relationship, and should consequently rank them 
^ distinct species. 

It is notorious on what excessively slight differences many 
palaeontologists have founded their species; and they do this 
the more readily if the specimens come from different sub- 
stages of the same formation. Some experienced concholo- 
gists are now sinking many of the very fine species of 
D'Orbigny and others into the rank of varieties; and on this 
view we do find the kind of evidence of change which on the 
theory we ought to find. Look again at the later tertiary de- 
posits, which include many shells believed by the majority of 
naturalists to be identical with existing species ; but some ex- 
cellent naturalists, as Agassiz and Pictet, maintain that all 
these tertiary species are specifically distinct, though the dis- 
tinction is admitted to be very slight; so that here, unless we 
believe that these eminent naturalists have been misled by 
their imaginations, and that these late tertiary species really 
present no difference whatever from their living representa- 
tives, or unless we admit, in opposition to the judgment of 
most naturalists, that these tertiary species are all truly dis- 
tinct from the recent, we have evidence of the frequent oc- 
currence of slight modifications of the kind required. If we 
look to rather wider intervals of time, namely, to distinct but 
consecutive stages of the same great formation, we find that 
the embedded fossils, though universally ranked as specific- 
ally different, yet are far more Josely related to each other 
than are the species found in more widely separated forma- 
tions; so that here again we have undoubted evidence of 


change in the direction required by the theory; but to this 
latter subject I shall return in the following chapter. 

With animals and plants that propagate rapidly and do not 
wander much, there is reason to suspect, as we have formerly 
seen, that their varieties are generally at first local ; and that 
such local varieties do not spread widely and supplant 
their parent-forms until they have been modified and per- 
fected in some considerable degree. According to this view, 
the chance of discovering in a formation in any one country 
all the early stages of transition between any two forms, is 
small, for the successive changes are supposed to have been 
local or confined to some one spot. Most marine animals 
have, a wide range; and we have seen that with plants it is 
those which have the widest range, that oftenest present va- 
rieties ; so that, with shells and other marine animals, it is 
probable that those which had the widest range, far exceed- 
ing the limits of the known geological formations in Europe, 
have oftenest given rise, first to local varieties and ultimately 
to new species; and this again would greatly lessen the 
chance of our being able to trace the stages of transition in 
any one geological formation. 

It is a more important consideration, leading to the same 
result, as lately insisted on by Dr. Falconer, namely, that the 
period during which each species underwent modification, 
though long as measured by years, w^as probably short in 
comparison with that during which it remained without un- 
dergoing any change. " 

It should not be forgotten, that at the present day, with 
perfect specimens for examination, two forms can seldom be 
connected by intermediate varieties, and thus proved to be 
the same species, until many specimens are collected from 
many places; and with fossil species this can rarely be done. 
We shall, perhaps, best perceive the improbability of our 
being enabled to connect species by numerous, fine, inter- 
mediate, fossil links, by asking ourselves whether, for in- 
stance, geologists at some future period will be able to prove 
that our different breeds of cattle, sheep, horses, and dogs are 
descended from a single stock or from several aboriginal 
stocks; or, again, whether certain sea-shells inhabiting the 
shores of North America, which are ranked by some con- 



chologists as distinct species from their European representa- 
tives, and by other conchologists as only varieties, are really 
varieties, or are, as it is called, specifically distinct. This 
could be effected by the future geologist only by his discov- 
ering in a fossil state numerous intermediate gradations; and 
such success is improbable in the highest degree. 

It has been asserted over and over again, by writers who 
believe in the immutability of species, that geology yields 
no linking forms. This assertion, as we shall see in the next 
chapter, is certainly erroneous. As Sir J. Lubbock has re- 
marked, "Every species is a link between other allied forms." 
If we take a genus having a score of species, recent and ex- 
tinct, and destroy four-fifths of them, no one doubts that the 
remainder will stand much more distinct from each other. 
If the extreme forms in the genus happen to have been thus 
destroyed, the genus itself will stand more distinct from 
other allied genera. What geological research has not re- 
vealed, is the former existence of infinitely numerous grada- 
tions, as fine as existing varieties, connecting together nearly 
all existing and extinct species. But this ought not to be ex- 
pected; yet this has been repeatedly advanced as a most 
serious objection against my views. 

It may be worth while to sum up the foregoing remarks on 
the causes of the imperfection of the geological record under 
an imaginary illustration. The Malay Archipelago is about 
the size of Europe from the North Cape to the Mediter- 
ranean, and from Britain to Russia ; and therefore equals all 
the geological formations which have been examined with any 
accuracy, excepting those of the United States of America. 
I fully agree with Mr. Godwin-Austen, that the present con- 
dition of the Malay Archipelago, with its numerous large 
islands separated by wide and shallow seas, probably repre- 
sents the former state of Europe, whilst most of our forma- 
tions were accumulating. The Malay Archipelago is one of 
the richest regions in organic beings; yet if all the species 
were to be collected which have ever lived there, how im- 
perfectly would they represent the natural history of the 
world ! 

But we have every reason to believe that the terrestrial 
productions of the archipelago would be preserved in an ex- 


tremely imperfect manner in the formations which we sup- 
pose to be there accumulating. Not many of the strictly 
littoral animals, or of those which lived on naked submarine 
rocks, would be embedded; and those embedded in gravel or 
sand would not endure to a distant epoch. Wherever sedi- 
ment did not accumulate on the bed of the sea, or where it 
did not accumulate at a sufficient rate to protect organic 
bodies from decay, no remains could be preserved. 

Formations rich in fossils of many kinds, and of thickness 
sufficient to last to an age as distant in futurity as the sec- 
ondary formations lie in the past, would generally be formed 
in the archipelago only during periods of subsidence. These 
periods of subsidence would be separated from each other 
by immense intervals of time, during which the area would 
be either stationary or rising; whilst rising, the fossiliferous 
formations on the steeper shores would be destroyed, almost 
as soon as accumulated, by the incessant coast-action, as we 
now see on the shores of South America. Even throughout 
the extensive and shallow seas within the archipelago, sedi- 
mentary beds could hardly be accumulated of great thickness 
during the periods of elevation, or become capped and pro- 
tected by subsequent deposits, so as to have a good chance of 
enduring to a very distant future. During the periods 
of subsidence, there would probably be much extinction 
of life; during the periods of elevation, there would be much 
variation, but the geological record would then be less 

It may be doubted whether the duration of any one great 
period of subsidence over the whole or part of the archipel- 
ago, together with a contemporaneous accumulation of sedi- 
ment, would exceed the average duration of the same specific 
forms ; and these contingencies are indispensable for the pres- 
ervation of all the transitional gradations between any two or 
more species. If such gradations were not all fully pre- 
served, transitional varieties would merely appear as so many 
new, though closely allied species. It is also probable thatf^ 
each great period of subsidence would be interrupted by os- 
cillations of level, and that slight climatal changes would I 
intervene during such lengthy periods ; and in these cases the \ 
inhabitants of the archipelago would migrate, and no closely \ 

L— lie XI I 


/ consecutive record of their modifications could be preserved 
L-in any one formation. 

Very many of the marine inhabitants of the archipelago 
now range thousands of miles beyond its confines ; and anal- 
ogy plainly leads to the belief that it would be chiefly these 
far-ranging species, though only some of them, which would 
oftenest produce new varieties ; and the varieties would at 
first be local or confined to one place, but if possessed of any 
decided advantage, or when further modified and improved, 
they would slowly spread and supplant their parent-forms. 
When such varieties returned to their ancient homes, as they 
would differ from their former state in a nearly uniform, 
though perhaps extremely slight degree, and as they would 
be found embedded in slightly different sub-stages of the 
same formation, they would, according to the principles fol- 
lowed by many palaeontologists, be ranked as new and distinct 
I If then there be some degree of truth in these remarks, we 
\ have no right to expect to find, in our geological formations, 
I an infinite number of those fine transitional forms which, on 
I our theory, have connected all the past and present species 
Lof the same group into one long and branching chain of life. 
r We ought only to look for a few links, and such assuredly 
I we do find — some more distantly, some more closely, related 
1 to each other; and these links, let them be ever so close, if 
\ found in different stages of the same formation, would, by 
Lmany palaeontologists, be ranked as distinct species. But I 
k do not pretend that I should ever have suspected how poor 
was the record in the best preserved geological sections, had 
not the absence of innumerable transitional links between the 
species which lived at the commencement and close of each 
formation, pressed so hardly on my theory. 


The abrupt manner in which whole groups of species sud- 
denly appear in certain formations, has been urged by several 
palaeontologists — for instance, by Agassiz, Pictet, and Sedg- 
jvick — as a fatal objection to the belief in the transmutation 


of species. If numerous species, belonging to the same gen- 
era or families, have really started into life at once, the fact 
would be fatal to the theory of evolution through natural 
selection. For the development by this means of a group of 
forms, all of which are descended from some one progenitor, 
must have been an extremely slow process ; and the progeni- 
tors must have lived long before their modified descendants. 
But we continually overrate the perfection of the geological 
record, and falsely infer, because certain genera or families 
have not been found beneath a certain stage, that they did 
not exist before that stage. In all cases positive palaeonto- 
logical evidence may be implicitly trusted; negative evidence 
is worthless, as experience has so often shown. We contin- 
ually forget how large the world is, compared with the area 
over which our geological formations have been carefully ex- 
amined ; we forget that groups of species may elsewhere have 
long existed, and have slowly multiplied, before they invaded 
the ancient archipelagoes of Europe and the United States. 
We do not make due allowance for the intervals of time 
which have elapsed between our consecutive formations, — ■ 
longer perhaps in many cases than the time required for the 
accumulation of each formation. These intervals will have 
given time for the multiplication of species from some one 
parent- form: and in the succeeding formation, such groups 
or species will appear as if suddenly created. 

I may here recall a remark formerly made, namely, that it 
might require a long succession of ages to adapt an organism 
to some new and peculiar line of life, for instance, to fly 
through the air; and consequently that the transitional forms 
would often long remain confined to some one region ; but 
that, when this adaptation had once been effected, and a few 
species had thus acquired a great advantage over other or- 
ganisms, a comparatively short time would be necessary to 
produce many divergent forms, which would spread rapidly 
and widely, throughout the world Professor Pictet, in his 
excellent Review of this work, in commenting on early 
transitional forms, and taking birds as an illustration, cannot 
see how the successive modifications of the anterior limbs of 
a supposed prototype could possibly have been of any advan- 
tage. But look at the penguins of the Southern Ocean ; have 


not these birds their front limbs in this precise intermediate 
state of "neither true arms nor true wings" ? Yet these birds 
hold their place victoriously in the battle for life; for they 
exist in infinite numbers and of many kinds. I do not sup- 
pose that we here see the real transitional grades through 
which the wings of birds have passed; but what special diffi- 
culty is there in believing that it might profit the modified 
descendants of the penguin, first to become enabled to flap 
along the surface of the sea like the logger-headed duck, and 
ultimately to rise from its surface and glide through the air? 
I will now give a few examples to illustrate the foregoing 
remarks, and to show how liable we are to error in supposing 
that whole groups of species have suddenly been produced. 
Even in so short an interval as that between the first and 
second editions of Pictet's great work on Palaeontology, pub- 
lished in 1844-46 and 1853-57, the conclusions on the first ap- 
pearance and disappearance of several groups of animals 
have been considerably modified; and a third edition would 
require still further changes. I may recall the well-known 
fact that in geological treatises, published not many years 
ago, mammals were always spoken of as having abruptly 
come in at the commencement of the tertiary series. And 
now one of the richest known accumulations of fossil mam- 
mals belongs to the middle of the secondary series ; and true 
mammals have been discovered in the new red sandstone at 
. nearly the commencement of this great series. Cuvier used 
to urge that no monkey occurred in any tertiary stratum ; but 
now extinct species have been discovered in India, South 
America, and in Europe, as far back as the miocene stage. 
Had it not been for the rare accident of the preservation of 
footsteps in the new red sandstone of the United States, who 
would have ventured to suppose that no less than at least 
thirty different bird-like animals, some of gigantic size, existed 
during that period? Not a fragment of bone has been dis- 
covered in these beds. Not long ago, palaeontologists main- 
tained that the whole class of birds came suddenly into ex- 
istence during the eocene period; but now we know, on the 
authority of Professor Owen, that a bird certainly lived dur- 
ing the deposition of the upper greensand ; and still more re- 
cently, that strange bird, the Archeopteryx, with a long 


lizard-like tail, bearing a pair of feathers on each joint, and 
with its wings furnished with two free claws, has been dis- 
covered in the oolitic slates of Solenhofcn. Hardly any recent 
discovery shows more forcibly than this, how little we as yet 
know of the former inhabitants of the world. 

I may give another instance, which, from having passed 
under my own eyes, has much struck me. In a memoir on 
Fossil Sessile Cirripedes, I stated that, from the large number 
of existing and extinct tertiary species ; from the extraordi- 
nary abundance of the individuals of many species all over 
the world, from the Arctic regions to the equator, inhabiting 
various zones of depths from the upper tidal limits to 50 
fathoms ; from the perfect manner in which specimens are 
preserved in the oldest tertiary beds ; from the ease with 
which even a fragment of a valve can be recognized ; from 
all these circumstances, I inferred that, had sessile cirripedes 
existed during the secondary periods, they would certainly 
have been preserved and discovered ; and as not one species 
had then been discovered in beds of this age, I concluded that 
this great group had been suddenly developed at the com- 
mencement of the tertiary series. This was a sore trouble 
to me, adding as I then thought one more instance of the 
abrupt appearance of a great group of species. But my work 
had hardly been published, when a skilful palaeontologist, M. 
Bosquet, sent me a drawing of a perfect specimen of an un- 
mistakeable sessile cirripede, which he had himself extracted 
from the chalk of Belgium. And, as if to make the case as 
striking as possible, this cirripede was a Chthamalus, a very 
common, large, and ubiquitous genus, of which not one 
species has as yet been found even in any tertiary stratum. 
Still more recently, a Pyrgoma, a member of a distinct sub- 
family of sessile cirripedes, has been discovered by Mr. 
Woodward in the upper chalk ; so that we now have abundant 
evidence of the existence of this group of animals during the 
secondary period. 

The case most frequently insisted on by palccontologists of 
the apparently sudden appearance of a whole group of species, 
is that of the teleostean fishes, low down, according to Agas- 
siz, in the Chalk period. This group includes the large ma- 
jority of existing species. But certain Jurassic and Triassic 


forms are now commonly admitted to be teleostean ; and even 
some palaeozoic forms have thus been classed by one high 
authority. If the teleosteans had really appeared suddenly in 
the northern hemisphere at the commencement of the chalk 
formation, the fact would have been highly remarkable; but 
it would not have formed an insuperable difficulty, vmless it 
could likewise have been shown that at the same period the 
species were suddenly and simultaneously developed in other 
quarters of the world. It is almost superfluous to remark 
that hardly any fossil-fish are known from south of the 
equator ; and by running through Pictet's Palaeontology it will 
be seen that very few species are known from several forma- 
tions in Europe. Some few families of fish now have a con- 
fined range; the teleostean fishes might formerly have had a 
similarly confined range, and after having been largely de- 
veloped in some one sea, have spread widely. Nor have we 
any right to suppose that the seas of the world have always 
been so freely open from south to north as they are at pres- 
ent. Even at this day, if the Malay Archipelago were con- 
verted into land, the tropical parts of the Indian Ocean would 
form a large and perfectly enclosed basin, in which any great 
group of marine animals might be multiplied; and here they 
would remain confined, until some of the species became 
adapted to a cooler climate, and were enabled to double the 
Southern capes of Africa or Australia, and thus reach other 
and distant seas. 

From these considerations, from our ignorance of the geol- 
ogy of other countries beyond the confines of Europe and the 
United States, and from the revolution in our palaeontological 
knowledge effected by the discoveries of the last dozen years, 
it seems to me to be about as rash to dogmatize on the suc- 
cession of organic forms throughout the world, as it would 
be for a naturalist to land for five minutes on a barren point 
in Australia, and then to discuss the number and range of its 



There is another and allied difficulty, which is much more 
serious. I allude to the manner in which species belonging 
to several of the main divisions of the animal kingdom sud- 
denly appear in the lowest known fossiliferous rocks. Most 
of the arguments which have convinced me that all the ex- 
isting species of the same group are descended from a single 
progenitor, apply with equal force to the earliest known 
species. For instance, it cannot be doubted that all the 
Cambrian and Silurian trilobites are descended from some 
one crustacean, which must have lived long before the Cam- 
brian age, and which probably differed greatly from any 
known animal. Some of the most ancient animals, as the 
Nautilus, Lingula, &c., do not differ much from living species; 
and it cannot on our theory be supposed, that these old spe- 
cies were the progenitors of all the species belonging to the 
same groups which have subsequently appeared, for they are 
not in any degree intermediate in character. 

Consequently, if the theory be true, it is indisputable that 
before the lowest Cambrian stratum was deposited long peri- 
ods elapsed, as long as, or probably far longer than, the whole 
interval from the Cambrian age to the present day ; and that 
during these vast periods the world swarmed with living 
creatures. Here we encounter a formidable objection; for it 
seems doubtful whether the earth, in a fit state for the habi- 
tation of living creatures, has lasted long enough. Sir W. 
Thompson concludes that the consolidation of the crust can 
hardly have occurred less than 20 or more than 400 million 
years ago, but probably not less than 98 or more than 200 
million years. These very wide limits show how doubtful 
the data are ; and other elements may have hereafter to be 
introduced into the problem. Mr. Croll estimates that about 
60 million years have elapsed since the Cambrian period, but 
this, judging from the small amount of organic change since 
the commencement of the Glacial epoch, appears a very short 
time for the many and great mutations of life, which have 
certainly occurred since the Cambrian formation ; and the 
previous 140 million years can hardly be considered as suffi- 


cient for the development of the varied forms of life which 
already existed during the Cambrian period. It is, however, 
probable, as Sir William Thompson insists, that the world at 
a very early period was subjected to more rapid and violent 
changes in its physical conditions than those now occurring; 
and such changes would have tended to induce changes at a 
corresponding rate in the organisms which then existed. 

To the question why we do not find rich fossiliferous de- 
posits belonging to these assumed earliest periods prior to the 
Cambrian system, I can give no satisfactory answer. Sev- 
eral eminent geologists, with Sir R. Murchison at their head, 
were until recently convinced that we beheld in the organic 
remains of the lowest Silurian stratum the first dawn of life. 
Other highly competent judges, as Lyell and E. Forbes, have 
disputed this conclusion. We should not forget that only a 
small portion of the world is known with accuracy. Not very 
long ago M. Barrande added another and lower stage, 
abounding with new and peculiar species, beneath the then 
known Silurian system ; and now, still lower down in the 
Lower Cambrian formation, Mr. Hicks has found in South 
Wales beds rich in trilobites, and containing various molluscs 
and annelids. The presence of phosphatic nodules and bitu- 
minous matter, even in some of the lowest azoic rocks, prob- 
ably indicates life at these periods ; and the existence of the 
Eozoon in the Laurentian formation of Canada is generally 
admitted. There are three great series of strata beneath the 
Silurian system in Canada, in the lowest of which the Eozoon 
is found. Sir W. Logan states that their "united thickness 
"may possibly far surpass that of all the succeeding rocks, 
"from the base of the palaeozoic series to the present time. We 
"are thus carried back to a period so remote that the appear- 
"ance of the so-called Primordial fauna (of Barrande) may 
"by some be considered as a comparatively modern event." 
The Eozoon belongs to the most lowly organised of all 
classes of animals, but is highly organised for its class; it 
existed in countless numbers, and, as Dr. Dawson has re- 
marked, certainly preyed on other minute organic beings, 
which must have lived in great numbers. Thus the words, 
which I wrote in 1859, about the existence of living beings 
long before the Cambrian period, and which are almost the 


same with those since used by Sir W. Logan, have proved 
true. Nevertheless, the difficuhy of assigning any good 
reason for the absence of vast piles of strata rich in fossils 
beneath the Cambrian system is very great. It does not seem 
probable that the most ancient beds have been quite worn 
away by denudation, or that their fossils have been wholly 
obliterated by metamorphic action, for if this had been the 
case we should have found only small remnants of the forma- 
tions next succeeding them in age, and these would always 
have existed in a partially metamorphosed condition. But 
the descriptions which we possess of the Silurian deposits 
over immense territories in Russia and in North America, do 
not support the view, that the older a formation is, the more 
invariably it has suffered extreme denudation and meta- 

The case at present must remain inexplicable ; and may 
be truly urged as a valid argument against the views here 
entertained. To show that it may hereafter receive some 
explanation, I will give the following hypothesis. From the 
nature of the organic remains which do not appear to have 
inhabited profound depths, in the several formations of 
Europe and of the United States; and from the amount of 
sediment, miles in thickness, of which the formations are 
composed, we may infer that from first to last large islands 
or tracts of land, whence the sediment vi^as derived, occurred 
in the neighbourhood of the now existing continents of 
Europe and North America. The same view has since been 
maintained by Agassiz and others. But we do not know 
what was the state of things in the intervals between the 
several successive formations; whether Europe and the 
United States during these intervals existed as dry land, or 
as a submarine surface near land, on which sediment was 
not deposited, or as the bed on an open and unfathomable sea. 

Looking to the existing oceans, which are thrice as exten- 
sive as the land, we see them studded with many islands ; 
but hardly one truly oceanic island (with the exception of 
New Zealand, if this can be called a truly oceanic island) is 
as yet knowm to afford even a remnant of any palaeozoic and 
secondary formation. Hence we may perhaps infer that 
during the palaeozoic and secondary periods, neither conti- 


nents nor continental islands existed where our oceans now 
extend; for had they existed, palaeozoic and secondary forma- 
tions would in all probability have been accumulated from 
sediment derived from their wear and tear ; and these would 
have been at least partially upheaved by the oscillations of 
level, which must have intervened during these enormously 
long periods. If then we may infer anything from these 
facts, we may infer that, where our oceans now extend, 
oceans have extended from the remotest period of which we 
have any record; and on the other hand, that where conti- 
nents now exist, large tracts of land have existed, subjected 
no doubt to great oscillations of level, since the Cambrian 
period. The colored map appended to my volume on Coral 
Reefs, led me to conclude that the great oceans are still 
mainly areas of subsidence, the great archipelagoes still areas 
of oscillations of level, and the continents areas of elevation. 
But we have no reason to assume that things have thus 
remained from the beginning of the world. Our continents 
seem to have been formed by a preponderance, during many 
oscillations of level, of the force of elevation; but may not 
the areas of preponderant movement have changed in the 
lapse of ages? At a period long antecedent to the Cambrian 
epoch, continents may have existed where oceans are now 
spread out; and clear and open oceans may have existed 
where our continents now stand. Nor should we be justified 
in assuming that if, for instance, the bed of the Pacific Ocean 
were now converted into a continent we should there find 
sedimentary formations in a recognisable condition older 
than the Cambrian strata, supposing such to have been for- 
merly deposited; for it might well happen that strata which 
had subsided some miles nearer to the centre of the earth, 
and which had been pressed on by an enormous weight of 
super-incumbent water, might have undergone far more 
metamorphic action than strata which have always remained 
nearer to the surface. The immense areas in some parts of the 
world, for instance in South America, of naked metamorphic 
rocks, which must have been heated under great pressure, 
have always seemed to me to require some special explana- 
tion ; and we may perhaps believe that we see in these large 
areas, the many formations long anterior to the Cambrian 


epoch in a completely metamorphosed and denuded con- 

The several difficulties here discussed, namely — that, 
though we find in our geological formations many links be- 
tween the species which now exist and which formerly 
existed, we do not find infinitely numerous fine transitional 
forms closely joining them all together; — the sudden man- 
ner in which several groups of species first appear in our 
European formations; — the almost entire absence, as at 
present known, of formations rich in fossils beneath the 
Cambrian strata, — are all undoubtedly of the most serious 
nature. We see this in the fact that the most eminent 
palaeontologists, namely, Cuvier, Agassiz, Barrande, Pictet, 
Falconer, E. Forbes, &c., and all our greatest geologists, as 
Lyell, Murchison, Sedgwick, &c., have unanimously, often 
vehemently, maintained the immutability of species. But 
Sir Charles Lyell now gives the support of his high author- 
ity to the other side ; and most geologists and palaeontologists 
are much shaken in their former belief. Those who believe 
that the geological record is in any degree perfect, will un- 
doubtedly at once reject the theory. For my part, follow- 
ing out Lyell's metaphor, I look at the geological record 
as a history of the world imperfectly kept, and written in 
a changing dialect; of this history we possess the last vol- 
ume alone, relating only to two or three countries. Of this 
volume, only here and there a short chapter has been pre- 
served; and of each page, only here and there a few lines. 
Each word of the slowly-changing language, more or less 
different in the successive chapters, may represent the forms 
of life, which are entombed in our consecutive formations, 
and which falsely appear to have been abruptly introduced. 
On this view, the difficulties above discussed are greatly 
diminished, or even disappear. 

On the Geological Succession of Organic Beings 

On the slow and successive appearance of new species — On their 
different rates of change — Species once lost do not reappear — 
Groups of species follow the same general rules in their appear- 
ance and disappearance as do single species — On extinction — 
On simultaneous changes in the forms of life throughout the 
world — On the affinities of extinct species to each other and to 
living species — On the state of development of ancient forms — 
On the succession of the same types within the same areas — 
Summary of preceding and present chapter. 

1ET US now see whether the several facts and laws relat- 
. ing to the geological succession of organic beings 
^ accord best with the common view of the immutability 
of species, or with that of their slow and gradual modifica- 
tion, through variation and natural selection. 

New species have appeared very slowly, one after another, 
both on the land and in the waters. Lyell has shown that 
it is hardly possible to resist the evidence on this head in the 
case of the several tertiary stages ; and every year tends 
to fill up the blanks between the stages, and to make the pro- 
portion between the lost and existing forms more gradual. 
In some of the most recent beds, though undoubtedly of high 
antiquity if measured by years, only one or two species are 
extinct, and only one or two are new, having appeared there 
for the first time, either locally, or, as far as we know, on 
the face of the earth. The secondary formations are more 
broken; but, as Bronn has remarked, neither the appear- 
ance nor disappearance of the many species embedded in 
each formation has been simultaneous. 

Species belonging to different genera and classes have not 
changed at the same rate, or in the same degree. In the 
older tertiary beds a few living shells may still be found in 
the midst of a multitude of extinct forms. Falconer has 



given a striking instance of a similar fact, for an existing 
crocodile is associated with many lost mammals and reptiles 
in the suh-Himalayan deposits. The Silurian Lingula differs 
but little from the living species of this genus ; whereas most 
of the other Silurian Molluscs and all the Crustaceans have 
changed greatly. The productions of the land seem to have 
changed at a quicker rate than those of the sea, of which 
a striking instance has been observed in Switzerland. There 
is some reason to believe that organisms high in the scale, 
change more quickly than those that are low: though there 
are exceptions to this rule. The amount of organic change, 
as Pictet has remarked, is not the same in each successive 
so-called formation. Yet if we compare any but the most 
closely related formations, all the species will be found to 
have undergone some change. When a species has once dis- 
appeared from the face of the earth, we have no reason to 
believe that the same identical form ever reappears. The 
strongest apparent exception to this latter rule is that of 
the so-called "colonies" of M. Barrande, which intrude for a 
period in the midst of an older formation, and then allow 
the pre-existing fauna to reappear; but Lyell's explanation, 
namely, that it is a case of temporary migration from a 
distinct geographical province, seems satisfactory. 

These several facts accord well with our theory, which 
includes no fixed law of development, causing all the in- 
habitants of an area to change abruptly, or simultaneously, 
or to an equal degree. The process of modification must be 
slow, and will generally affect only a few species at the 
same time ; for the variability of each species is independent 
of that of all others. Whether such variations or individual 
differences as may arise will be accumulated through natural 
selection in a greater or less degree, thus causing a greater 
or less amount of permanent modification, will depend on 
many complex contingencies — on the variations being of a 
beneficial nature, on the freedom of intercrossing, on the 
slowly changing physical conditions of the country, on the 
immigration of new colonists, and on the nature of the other 
inhabitants with which the varying species come into com- 
petition. Hence it is by no means surprising that one species 
should retain the same identical form much longer than 


others; or, if changing, should change in a less degree. We 
find similar relations between the existing inhabitants of dis- 
tinct countries ; for instance, the land-shells and coleopterous 
insects of Madeira have come to differ considerably from 
their nearest allies on the continent of Europe, whereas the 
marine shells and birds have remained unaltered. We can 
perhaps understand the apparently quicker rate of change in 
terrestrial and in more highly organised productions com- 
pared with marine and lower productions, by the more com- 
plex relations of the higher beings to their organic and in- 
organic conditions of life, as explained in a former chapter. 
When many of the inhabitants of any area have become 
modified and improved, we can understand, on the principle 
of competition, and from the all-important relations of or- 
ganism to organism in the struggle for life, that any form 
which did not become in some degree modified and improved, 
would be liable to extermination. Hence we see why all 
the species in the same region do at last, if we look to long 
enough intervals of time, become modified, for otherwise 
they would become extinct. 

In members of the same class the average amount of 
change during long and equal periods of time, may, perhaps, 
be nearly the same; but as the accumulation of enduring 
formation, rich in fossils, depends on great masses of sedi- 
ment being deposited on subsiding areas, our formations have 
been almost necessarily accumulated at wide and irregularly 
intermittent intervals of time; consequently the amount of 
organic change exhibited by the fossils embedded in consecu- 
tive formations is not equal. Each formation, on this view, 
does not mark a new and complete act of creation, but only 
an occasional scene, taken almost at hazard in an ever 
slowly changing drama. 

We can clearly understand why a species when once lost 
should never reappear, even if the very same conditions of 
life, organic and inorganic, should recur. For though the 
offspring of one species might be adapted (and no doubt 
this has occurred in innumerable instances) to fill the place 
of another species in the economy of nature, and thus sup- 
plant it; yet the two forms — the old and the new — would 
not be identically the same; for both would almost certainly 


inherit different characters from their distinct progenitors; 
and organisms already differing would vary in a different 
manner. For instance, it is possible, if all our fantail 
pigeons were destroyed, that fanciers might make a new 
breed hardly distinguishable from the present breed ; but 
if the parent rock-pigeon were likewise destroyed, and under 
nature we have every reason to believe that parent-forms 
are generally supplanted and exterminated by their improved 
offspring, it is incredible that a fantail, identical with the 
existing breed, could be raised from any other species of 
pigeon, or even from any other well-established race of the 
domestic pigeon, for the successive variations would almost 
certainly be in some degree different, and the newly-formed 
variety would probably inherit from its progenitor some char- 
acteristic differences. 

Groups of species, that is, genera and families, follow the 
same general rules in their appearance and disappearance as 
do single species, changing more or less quickly, and in a 
greater or lesser degree. A group, when it has once dis- 
appeared, never reappears ; that is, its existence, as long as it 
lasts, is continuous. I am aware that there are some ap- 
parent exceptions to this rule, but the exceptions are surpris- 
ingly few, so few that E. Forbes, Pictet, and Woodward 
(though all strongly opposed to such views as I maintain) 
admit its truth ; and the rule strictly accords with the theory. 
For all the species of the same group, however long it may 
have lasted, are the modified descendants one from the other, 
and all from a common progenitor. In the genus Lingula, 
for instance, the species which have successively appeared at all 
ages must have been connected by an unbroken series of gen- 
erations, from the lowest Silurian stratum to the present day. 

We have seen in the last chapter that whole groups of 
species sometimes falsely appear to have been abruptly devel- 
oped; and I have attempted to give an explanation of this 
fact, which if true would be fatal to my views. But such 
cases are certainly exceptional ; the general rule being a 
gradual increase in number, until the group reaches its maxi- 
mum, and then, sooner or later, a gradual decrease. If the 
number of the species included within a genus, or the number 
of the genera within a family, be represented by a vertical 


line of varying thickness, ascending through the successive 
geological formations, in which the species are found, the 
line will sometimes falsely appear to begin at its lower end, 
not in a sharp point, but abruptly ; it then gradually thickens 
upwards, often keeping of equal thickness for a space, and 
ultimately thins out in the upper beds, marking the decrease 
and final extinction of the species. This gradual increase in 
number of the species of a group is strictly conformable 
with the theory, for the species of the same genus, 
and the genera of the same family, can increase only slowly 
and progressively ; the process of modification and the pro- 
duction of a number of allied forms necessarily being a slow 
and gradual process, — one species first giving rise to two 
or three varieties, these being slowly converted into species, 
•which in their turn produce by equally slow steps other 
varieties and species, and so on, like the branching of a great 
tree from a single stem, till the group becomes large. 


We have as yet only spoken incidentally of the disappear- 
ance of species and of groups of species. On the theory of 
natural selection, the extinction of old forms and the pro- 
duction of new and improved forms are intimately con- 
nected together. The old notion of all the inhabitants of the 
earth having been swept away by catastrophes at successive 
periods is very generally given up, even by those geologists, 
as Elie de Beaumont, Murchison, Barrande, &c., whose gen- 
eral views would naturally lead them to this conclusion. 
On the contrary, we have every reason to believe, from the 
study of the tertiary formations, that species and groups of 
species gradually disappear, one after another, first from one 
spot, then from another, and finally from the world. In 
some few cases, however, as by the breaking of an isthmus 
and the consequent irruption of a multitude of new inhabi- 
tants into an adjoining sea, or by the final subsidence of an 
island, the process of extinction may have been rapid. Both 
single species and whole groups of species last for very un- 
equal periods; some groups, as we have seen, have endured 
from the earliest known dawn of life to the present day; 


some have disappeared before the close of the palaeozoic 
period. No fixed law seems to determine the length of time 
during which any single species or any single genus en- 
dures. There is reason to believe that the extinction of a 
whole group of species is generally a slower process than 
their production: if their appearance and disappearance be 
represented, as before, by a vertical line of varying thickness 
the line is found to taper more gradually at its upper end, 
which marks the progress of extermination, than at its 
lower end, which marks the first appearance and the early 
increase in number of the species. In some cases, however, 
the extermination of whole groups, as of ammonites, towards 
the close of the secondary period, has been wonderfully 

The extinction of species has been involved in the most 
gratuitous mystery. Some authors have even supposed that, 
as the individual has a definite length of life, so have species 
a definite duration. No one can have marvelled more than I 
have done at the extinction of species. When I found in La 
Plata the tooth of a horse embedded with the remains of 
Mastodon, Megatherium, Toxodon, and other extinct mon- 
sters, which all co-existed with still living shells at a very 
late geological period, I was filled with astonishment; for, 
seeing that the horse, since its introduction by the Span- 
iards into South America, has run wild over the whole coun- 
try and has increased in numbers at an unparalleled rate, I 
asked myself what could so recently have exterminated the 
former horse under conditions of life apparently so favour- 
able. But my astonishment was groundless. Professor 
Owen soon perceived that the tooth, though so like that of 
the existing horse, belonged to an extinct species. Had this 
horse been still living, but in some degree rare, no naturalist 
would have felt the least surprise at its rarity; for rarity 
is the attribute of a vast number of species of all classes, in 
all countries. If we ask ourselves why this or that species 
is rare, we answer that something is unfavourable in its 
conditions of life; but what that something is we can hardly 
ever tell. On the supposition of the fossil horse still existing 
as a rare species, we might have felt certain, from the 
analogy of all other mammals, even of the slow-breeding 


elephant, and from the history of the naturalisation of 
the domestic horse in South America, that under more 
favourable conditions it would in a very few years have 
stocked the whole continent. But we could not have told 
what the unfavourable conditions were which checked its in- 
crease, whether some one or several contingencies, and at 
what period of the horse's life, and in what degree they 
severally acted. If the conditions had gone on, however 
slowly, becoming less and less favourable, we assuredly 
should not have perceived the fact, yet the fossil horse would 
certainly have become rarer and rarer, and finally extinct; 
— its place being seized on by some more successful com- 

It is most difficult always to remember that the increase 
of every creature is constantly being checked by unperceived 
hostile agencies; and that these same unperceived agencies 
are amply sufficient to cause rarity, and finally extinction. 
So little is this subject understood, that I have heard sur- 
prise repeatedly expressed at such great monsters as the 
Mastodon and the more ancient Dinosaurians having be- 
come extinct; as if mere bodily strength gave victory in the 
battle of life. Mere size, on the contrary, would in some 
cases determine, as has been remarked by Owen, quicker 
extermination from the greater amount of requisite food. 
Before man inhabited India or Africa, some cause must 
have checked the continued increase of the existing ele- 
phant. A highly capable judge, Dr. Falconer, believes that 
it is chiefly insects which, from incessantly harassing and 
weakening the elephant in India, check its increase; and 
this was Bruce's conclusion with respect to the African ele- 
phant in Abyssinia. It is certain that insects and blood- 
sucking bats determine the existence of the larger natural- 
ized quadrupeds in several parts of S. America. 
^ We see in many cases in the more recent tertiary forma- 
tions, that rarity precedes extinction ; and we know that this 
has been the progress of events with those animals which 
have been exterminated, either locally or wholly, through 
man's agency. I may repeat what I published in 1845, 
namely, that to admit that species generally become rare 
before they become extinct — to feel no surprise at the rarity 


of a species, and yet to marvel greatly when the species 
ceases to exist, is much the same as to admit that sickness 
in the individual is the forerunner of death — to feel no sur- 
prise at sickness, but, when the sick man dies, to wonder and 
to suspect that he died by some deed of violence. 

The theory of natural selection is grounded on the belief 
that each new variety and ultimately each new species, is 
produced and maintained by having some advantage over 
those with which it comes into competition; and the conse- 
quent extinction of the less favoured forms almost inevitably 
follows. It is the same with our domestic productions ; when 
a new and slightly improved variety has been raised, it at 
first supplants the less improved varieties in the same neigh- 
bourhood; when much improved it is transported far and 
near, like our short-horn cattle, and takes the place of other 
breeds in other countries. Thus the appearance of new 
forms and the disappearance of old forms, both those natu- 
rally and those artificially produced, are bound together. In 
flourishing groups, the number of new specific forms which 
have been produced within a given time has at some periods 
probably been greater than the number of the old specific 
forms which have been exterminated ; but we know that spe- 
cies have not gone on indefinitely increasing, at least during 
the later geological epochs, so that, looking to later times, 
we may believe that the production of new forms has caused 
the extinction of about the same number of old forms. 

The competition will generally be most severe, as formerly 
explained and illustrated by examples, between the forms 
which are most like each other in all respects. Hence the 
improved and modified descendants of a species will gener- 
ally cause the extermination of the parent species; and if 
many new forms have been developed from any one species, 
the nearest allies of that species, i.e., the species of the same 
genus, will be the most liable to extermination. Thus, as I 
believe, a number of new species descended from one species, 
that is a new genus, comes to supplant an old genus, belong- 
ing to the same family. But it must often have happened 
that a new species belonging to some one group has seized 
on the place occupied by a species belonging to a distinct 
group, and thus have caused its extermination. If many 


allied forms be developed from the successful intruder, many 
will have to yield their places; and it will generally be the 
allied forms, which will suffer from some inherited inferior- 
ity in common. But whether it be species belonging to the 
same or to a distinct class, which have yielded their places 
to other modified and improved species, a few of the sufferers 
may often be preserved for a long time, from being fitted to 
some peculiar line of life, or from inhabiting some distant 
and isolated station, where they will have escaped severe 
competition. For instance, some species of Trigonia, a great 
genus of shells in the secondary formations, survive in the 
Australian seas; and a few members of the great and almost 
extinct group of Ganoid fishes still inhabit our fresh waters. 
Therefore the utter extinction of a group is generally, as 
we have seen, a slower process than its production. 

With respect to the apparently sudden extermination of 
whole families or orders, as of Trilobites at the close of the 
palaeozoic period and of Ammonites at the close of the sec- 
ondary period, we must remember what has been already 
said on the probable wide intervals of time between our con- 
secutive formations; and in these intervals there may have 
been much slow extermination. Moreover, when, by sud- 
den immigration or by unusually rapid development, many 
species of a new group have taken possession of an area, 
many of the older species will have been exterminated in a 
correspondingly rapid manner; and the forms which thus 
yield their places will commonly be allied, for they will par- 
take of the same inferiority in common. 

Thus, as it seems to me, the manner in which single species 
and whole groups of species become extinct accord well with 
the theory of natural selection. We need not marvel at ex- 
tinction ; if we must marvel, let it be at our own presumption 
in imagining for a moment that we understand the many 
complex contingencies on which the existence of each spe- 
cies depends. If we forget for an instant that each species 
tends to increase inordinately, and that some check is always 
in action, yet seldom perceived by us, the whole economy of 
nature will be utterly obscured. Whenever we can precisely 
say why this species is more abundant in individuals than 
that; why this species and not another can be naturalised in 


a given country; then, and not until then, we may justly feci 
surprise why we cannot account for the extinction of any 
particular species or group of species. 


Scarcely any palseontological discovery is more striking 
than the fact that the forms of life change almost simulta- 
neously throughout the world. Thus our European Chalk 
formation can be recognised in many distinct regions, under 
the most different climates, where not a fragment of the 
mineral chalk itself can be found; namely in North America, 
in equatorial South America, in Ticrra del Fuego, at the 
Cape of Good Hope, and in the peninsula of India. For at 
these distant points, the organic remains in certain beds pre- 
sent an unmistakeable resemblance to those of the Chalk. It 
is not that the same species are met with ; for in some cases 
not one species is identically the same, but they belong to the 
same families, genera, and sections of genera, and sometimes 
are similarly characterised in such trifling points as mere 
superficial sculpture. Moreover, other forms, which are not 
found in the Chalk of Europe, but which occur in the forma- 
tions either above or below, occur in the same order at these 
distant points of the world. In the several successive palaeo- 
zoic formations of Russia, Western Europe, and North 
America, a similar parallelism in the forms of life has been 
observed by several authors; so it is, according to Lyell, with 
the European and North American tertiary deposits. Even 
if the few fossil species which are common to the Old and 
New Worlds were kept wholly out of view, the general par- 
allelism in the successive forms of life, in the palaeozoic and 
tertiary stages, would still be manifest, and the several for- 
mations could be easily correlated. 

These observations, however, relate to the marine inhabi- 
tants of the world: we have not sufficient data to judge 
whether the productions of the land and of fresh water at 
distant points change in the same parallel manner. We may 
doubt whether they have thus changed: if the Megatherium, 
Mylodon, Macrauchenia, and Toxodon had been brought to 


Europe from La Plata, without any information in regard 
to their geological position, no one would have suspected 
that they had co-existed with sea-shells all still living; but 
as these anomalous monsters co-existed with the Mastodon 
and Horse, it might at least have been inferred that they 
had lived during one of the later tertiary stages. 

When the marine forms of life are spoken of as having 
changed simultaneously throughout the world, it must not be 
supposed that this expression relates to the same year, or to 
the same country, or even that it has a very strict geological 
sense; for if all the marine animals now living in Europe, 
and all those that lived in Europe during the pleistocene 
period (a very remote period as measured by years, includ- 
ing the whole glacial epoch) were compared with those now 
existing in South America or in Australia, the most skilful 
naturalist would hardly be able to say whether the present 
or the pleistocene inhabitants of Europe resembled most 
closely those of the southern hemisphere. So, again, several 
highly competent observers maintain that the existing pro- 
ductions of the United States are more closely related to 
those which lived in Europe during certain late tertiary 
stages, than to the present inhabitants of Europe; and if this 
be so, it is evident that fossiliferous beds now deposited on 
the shores of North America would hereafter be liable to be 
classed with somewhat older European beds. Nevertheless, 
looking to a remotely future epoch, there can be little doubt 
that all the more modern marine formations, namely, the 
upper pliocene, the pleistocene and strictly modern beds of 
Europe, North and South America, and Australia, from con- 
taining fossil remains in some degree allied, and from not 
including those forms which are found only in the older 
underlying deposits, would be correctly ranked as simulta- 
neous in a geological sense. 

The fact of the forms of life changing simultaneously, in 
the above large sense, at distant parts of the world, has 
greatly struck those admirable observers, MM. de Verneuil 
and d'Archiac. After referring to the parallelism of the 
palaeozoic forms of life in various parts of Europe, they add, 
"If, struck by this strange sequence, we turn our attention 
to North America, and there discover a series of analogous 


phenomena, it will appear certain that all these modifications 
of species, their extinction, and the introduction of new ones, 
cannot be owing to mere changes in marine currents or other 
causes more or less local and temporary, but depend on gen- 
eral laws which govern the whole animal kingdom." M. 
Barrande has made forcible remarks to precisely the same 
effect. It is, indeed, quite futile to look to changes of cur- 
rents, climate, or other physical conditions, as the cause of 
these great mutations in the forms of life throughout the 
world, under the most different climates. We must, as Bar- 
rande has remarked, look to some special law. We shall see 
this more clearly when we treat of the present distribution 
of organic beings, and find how slight is the relation between 
the physical conditions of various countries and the nature 
of their inhabitants. 

This great fact of the parallel succession of the forms of 
life throughout the world, is explicable on the theory of 
natural selection. New species arc formed by having some 
advantage over older forms ; and the forms, which are al- 
ready dominant, or have some advantage over the other 
forms in their own country, give birth to the greatest num- 
ber of new varieties or incipient species. We have distinct 
evidence on this head, in the plants which are dominant, that 
is, which are commonest and most widely diffused, producing 
the greatest number of new varieties. It is also natural that 
the dominant, varying, and far-spreading species, which have 
already invaded to a certain extent the territories of other 
species, should be those which would have the best chance 
of spreading still further, and of giving rise in new countries 
to other new varieties and species The process of diffusion 
would often be very slow, depending on climatal and geo- 
graphical changes, on strange accidents, and on the gradual 
acclimatisation of new species to the various climates 
through which they might have to pass, but in the course 
of time the dominant forms would generally succeed in 
spreading and would ultimately prevail. The diffusion 
would, it is probable, be slower with the terrestrial inhabi- 
tants of the distinct continents than with the marine inhabi- 
tants of the continuous sea. We might therefore expect to 
find, as we do find, a less strict degree of parallelism in the 


succession of the productions of the land than with those of 

the sea. j 

Thus, as it seems to me, the parallel, and, taken in a large I 

sense, simultaneous, succession of the same forms of life | 

throughout the world, accords well with the principle of new 
species having been formed by dominant species spreading 
widely and varying; the new species thus produced being 
themselves dominant, owing to their having had some ad- 
vantage over their already dominant parents, as well as over 
other species, and again spreading, varying, and producing 
new forms. The old forms which are beaten and which 
yield their places to the new and victorious forms, will gen- 
erally be allied in groups, from inheriting some inferiority 
in common; and therefore, as new and improved groups 
spread throughout the world, old groups disappear from the 
world; and the succession of forms everywhere tends to 
correspond both in their first appearance and final disappear- 

There is one other remark connected with this subject 
worth making. I have given my reasons for believing that 
most of our great formations, rich in fossils, were deposited 
during periods of subsidence; and that blank intervals of 
vast duration, as far as fossils are concerned, occurred dur- 
ing the periods when the bed of the sea was either stationary 
or rising, and likewise when sediment was not thrown down 
quickly enough to embed and preserve organic remains. 
During these long and blank intervals I suppose that the in- 
habitants of each region underwent a considerable amount 
of modification and extinction, and that there was much 
migration from other parts of the world. As we have rea- 
son to believe that large areas are affected by the same move- 
ment, it is probable that strictly contemporaneous formations 
have often been accumulated over very wide spaces in the 
same quarter of the world ; but we are very far from having 
any right to conclude that this has invariably been the case, 
and that large areas have invariably been affected by the 
same movements. When two formations have been deposited 
in two regions during nearly, but not exactly, the sarne 
period, we should find in both, from the causes explained in 
the foregoing paragraphs, the same general succession in 


the forms of life; but the species would not exactly corre- 
spond ; for there will have been a little more time in the one 
region than in the other for modification, extinction, and 

I suspect that cases of this nature occur in Europe. Mr. 
Prestwich, in his admirable Memoirs on the eocene deposits 
of England and France, is able to draw a close general par- 
allelism between the successive stages in the two countries; 
but when he compares certain stages in England with those 
in France, although he finds in both a curious accordance in 
the numbers of the species belonging to the same genera, yet 
the species themselves differ in a manner very difficult to 
account for considering the proximity of the two areas, — ■ 
unless, indeed, it be assumed that an isthmus separated two 
seas inhabited by distinct, but contemporaneous, faunas. 
Lyell has made similar observations on some of the later ter- 
tiary formations. Barrande, also, shows that there is a strik- 
ing general parallelism in the successive Silurian deposits of 
Bohemia and Scandinavia; nevertheless he finds a surprising 
amount of difference in the species. If the several forma- 
tions in these regions have not been deposited during the 
same exact periods, — a formation in one region often cor- 
responding with a blank interval in the other, — and if in 
both regions the species have gone on slowly changing dur- 
ing the accumulation of the several formations and during 
the long intervals of time between them; in this case the sev- 
eral formations in the two regions could be arranged in the 
same order, in accordance with the general succession of the 
forms of life, and the order would falsely appear to be 
strictly parallel ; nevertheless the species would not be all 
the same in the apparently corresponding stages in the two 


Let us now look to the mutual affinities of extinct and 
living species. All fall into a few grand classes; and this 
fact is at once explained on the principle of descent. The 
more ancient any form is, the more, as a general rule, it dif- 


fers from living forms. But, as Buckland long ago re- 
marked, extinct species can all be classed either in still ex- 
isting groups, or between them. That the extinct forms of 
life help to fill up the intervals between existing genera, 
families, and orders, is certainly true; but as this statement 
has often been ignored or even denied, it may be well to 
make some remarks on this subject, and to give some in- 
stances. If we confine our attention either to the living or 
to the extinct species of the same class, the series is far less 
perfect that if we combine both into one general system. In 
the writings of Professor Owen we continually meet with 
the expression of generalised forms, as applied to extinct 
animals; and in the writings of Agassiz, of prophetic or syn- 
thetic types; and these terms imply that such forms are in 
fact intermediate or connecting links. Another distinguished 
palaeontologist, M. Gaudry, has shown in the most striking 
manner that many of the fossil mammals discovered by him 
in Attica serve to break down the intervals between existing 
genera. Cuvier ranked the Ruminants and Pachyderms, as 
two of the most distinct orders of mammals: but so many 
fossil links have been disentombed that Owen has had to 
alter the whole classification, and has placed certain pachy- 
derms in the same sub-order with ruminants ; for example, he 
dissolves by gradations the apparently wide interval between 
the pig and the camel. The Ungulata or hoofed quadrupeds 
are now divided into the even-toed or odd-toed divisions; 
but the Macrauchenia of S. America connects to a certain 
extent these two grand divisions. No one will deny that 
the Hipparion is intermediate between the existing horse 
and certain older ungulate forms. What a wonderful con- 
necting link in the chain of mammals is the Typotherium 
from S. America, as the name given to it by Professor Ger- 
vais expresses, and which cannot be placed in any existing 
order. The Sirenia form a very distinct group of mammals, 
and one of the most remarkable peculiarities in the existing 
dugong and lamentin is the entire absence of hind limbs 
without even a rudiment being left; but the extinct Hali- 
therium had, according to Professor Flower, an ossified 
thigh-bone "articulated to a well-defined acetabulum in the 
pelvis," and it thus makes some approach to ordinary hoofed 


quadrupeds, to which the Sirenia are in other respects allied. 
I'he cetaceans or whales are widely different from all other 
mammals, but the tertiary Zeuglodon and Squalodon, which 
have been placed by some naturalists in an order by them- 
selves, are considered by Professor Huxley to be undoubt- 
edly cetaceans, ''and to constitute connecting links with the 
aquatic carnivora." 

Even the wide interval between birds and reptiles has been 
shown by the naturalist just quoted to be partially bridged 
over in the most unexpected manner, on the one hand, by the 
ostrich and extinct Archeopteryx, and on the other hand, by 
the Compsognathus, one of the Dinosaurians — that group 
which includes the most gigantic of all terrestrial reptiles. 
Turning to the Invertebrata, Barrande asserts, a higher au- 
thority could not be named, that he is every day taught that, 
although palaeozoic animals can certainly be classed under 
existing groups, yet that at this ancient period the groups 
were not so distinctly separated from each other as they 
now are. 

Some writers have objected to any extinct species, or 
group of species, being considered as intermediate between 
any two living species, or groups of species. If by this term 
Jt is meant that an extinct form is directly intermediate in 
all its characters between two living forms or groups, the 
objection is probably valid. But in a natural classification 
many fossil species certainly stand between living species, 
and some extinct genera between living genera, even be- 
tween genera belonging to distinct families. The most com- 
mon case, especially with respect to very distinct groups, 
such as fish and reptiles, seems to be, that, supposing them 
to be distinguished at the present day by a score of char- 
acters, the ancient members are separated by a somewhat 
lesser number of characters; so that the two groups formerly 
made a somewhat nearer approach to each other than they 
now do. 

It is a common belief that the more ancient a form is, by 
so much the more it tends to connect by some of its char- 
acters groups now widely separated from each other. This 
remark no doubt must be restricted to those groups which 
have undergone much change in the course of geological 


ages ; and it would be difficult to prove the truth of the propo- 
sition, for every now and then a living animal, as the Lepi- 
dosiren, is discovered having affinities directed towards very 
distinct groups. Yet if we compare the older Reptiles and 
Batrachians, the older Fish, the older Cephalopods, and the 
eocene Mammals, with the more recent members of the same 
classes, we must admit that there is truth in the remark. 

Let us see how far these several facts and inferences ac- 
cord with the theory of descent with modification. As the 
subject is somewhat complex, I must request the reader to 
turn to the diagram in the fourth chapter. We may suppose 
that the numbered letters in italics represent genera, and the 
dotted lines diverging from them the species in each genus. 
The diagram is much too simple, too few genera and too few 
species being given, but this is unimportant for us. The 
horizontal lines may represent successive geological forma- 
tions, and all the forms beneath the uppermost line may be 
considered as extinct. The three existing genera o" q^\ p^\ 
will form a small family; t" and /" a closely allied family 
or sub-family; and o'\ e'\ m" a third family. These three 
families, together with the many extinct genera on the sev- 
eral lines of descent diverging from the parent-form (A) 
will form an order, for all will have inherited something in 
common from their ancient progenitor. On the principle of 
the continued tendency to divergence of character, which 
was formerly illustrated by this diagram, the more recent 
any form is, the more it will generally differ from its ancient 
progenitor. Hence we can understand the rule that the most 
ancient fossils differ most from existing forms. We must 
not, however, assume that divergence of character is a neces- 
sary contingency; it depends solely on the descendants from 
a species being thus enabled to seize on many and different 
places in the economy of nature. Therefore it is quite pos- 
sible, as we have seen in the case of some Silurian forms, 
that a species might go on being slightly modified in relation 
to its slightly altered conditions of life, and yet retain 
throughout a vast period the same general characteristics. 
This is represented in the diagram by the letter f". 

All the many forms, extinct and recent, descended from 
(A), make, as before remarked, one order; and this order. 


from the continued effects of extinction and divergence of 
character, has become divided into several sub-families and 
families, some of which are supposed to have perished at 
different periods, and some to have endured to the present 

By looking at the diagram we can see that if many of the 
extinct forms supposed to be imbedded in the successive 
formations, were discovered at several points low down in 
the series, the three existing families on the uppermost line 
would be rendered less distinct from each other. If, for in- 
stance, the genera a\ d" , a", /", »^^ »»", 7n', were disinterred, 
these three families would be so closely linked together that 
they probably would have to be united into one great fam- 
ily, in nearly the same manner as has occurred with rumi- 
nants and certain pachyderms. Yet he who objected to con- 
sider as intermediate the extinct genera, which thus link 
together the living genera of three families, would be partly 
justified, for they are intermediate, not directly, but only by 
a long and circuitous course through many widely dift'ercnt 
forms. If many extinct forms were to be discovered above 
one of the horizontal lines or geological formations — for in- 
stance, above No. VI. — but none from beneath this line, then 
only two of the families (those on the left hand, a\ &c., and 
fc," &c.) would have to be united into one; and there would 
remain two families, which would be less distinct from each 
other than they were before the discovery of the fossils. 
So again if the three families formed of eight genera (a" to 
*»"), on the uppermost line, be supposed to differ from each 
other by half-a-dozen important characters, then the fami- 
lies which existed at the period marked VI. would certainly 
have differed from each other by a less number of char- 
acters; for they would at this early stage of descent have 
diverged in a less degree from their common progenitor. 
Thus it comes that ancient and extinct genera are often in a 
greater or less degree intermediate in character between 
their modified descendants, or between their collateral 

Under nature the process will be far more complicated 
than is represented in the diagram; for the groups will have 
been more numerous; they will have endured for extremely 


unequal lengths of time, and will have been modified in vari- 
ous degrees. As we possess only the last volume of the geo- 
logical record, and that in a very broken condition, we have 
no right to expect, except in rare cases, to fill up the wide 
intervals in the natural system, and thus to unite distinct 
families or orders. All that we have a right to expect is, 
that those groups which have, within known geological peri- 
ods, undergone much modification, should in the older for- 
mations make some slight approach to each other; so that 
the older members should differ less from each other in some 
of their characters than do the existing members of the 
same groups; and this by the concurrent evidence of our best 
palaeontologists is frequently the case. 

Thus, on the theory of descent with modification, the main 
facts with respect to the mutual affinities of the extinct 
forms of life to each other and to living forms, are explained 
in a satisfactory manner. And they are wholly inexplicable 
on any other view. 

On this same theory, it is evident that the fauna during 
any one great period in the earth's history will be inter- 
mediate in general character between that which preceded 
and that which succeeded it. Thus the species which lived 
at the sixth great stage of descent in the diagram are the 
modified offspring of those which lived at the fifth stage, 
and are the parents of those which became still more modi- 
fied at the seventh stage; hence they could hardly fail to be 
nearly intermediate in character between the forms of life 
above and below. We must, however, allow for the entire 
extinction of some preceding forms, and in any one region 
for the immigration of new forms from other regions, and 
for a large amount of modification during the long and blank 
interval between the successive formations. Subject to these 
allowances, the fauna of each geological period undoubtedly 
is intermediate in character, between the preceding and suc- 
ceeding faunas. I need give only one instance, namely, the 
manner in which the fossils of the Devonian system, when 
this system was first discovered, were at once recognised by 
palaeontologists as intermediate in character between those 
of the overlying carboniferous, and underlying Silurian sys- 
tems. But each fauna is not necessarily exactly intermediate, 


as unequal intervals of time have elapsed between consecM- 
tive formations. 

It is no real objection to the truth of the statement that 
the fauna of each period as a whole is nearly intermediate 
in character between the preceding and succeeding faunas, 
that certain genera offer exceptions to the rule. For in- 
stance, the species of mastodons and elephants, when ar- 
ranged by Dr. Falconer in two series, — in the first place 
according to their mutual affinities, and in the second place 
according to their periods of existence, — do not accord in 
arrangement. The species extreme in character are not the 
oldest or the most recent; nor are those which are interme- 
diate in character, intermediate in age. But supposing for 
an instant, in this and other such cases, that the record of 
the first appearance and disappearance of the species was 
complete, which is far from the case, we have no reason to 
believe that forms successively produced necessarily endure 
for corresponding lengths of time. A very ancient form 
may occasionally have lasted much longer than a form else- 
where subsequently produced, especially in the case of terres- 
trial productions inhabiting separated districts. To compare 
small things with great; if the principal living and extinct 
races of the domestic pigeon were arranged in serial affinity, 
this arrangement would not closely accord with the order in 
time of their production, and even less with the order of 
their disappearance; for the parent, rock-pigeon still lives; 
and many varieties between the rock-pigeon and the carrier 
have become extinct; and carriers which are extreme in the 
important character of length of back originated earlier than 
short-beaked tumblers, which are at the opposite end of the 
series in this respect. 

Closely connected with the statement, that the organic re- 
mains from an intermediate formation are in some degree 
intermediate in character, is the fact, insisted on by all 
palaeontologists, that fossils from two consecutive formations 
are far more closely related to each other, than are the fos- 
sils from two remote formations. Pictet gives as a well- 
known instance, the general resemblance of the organic re- 
mains from the several stages of the Chalk formation, 
though the species arc distinct in each stage. This fact 


alone, from. its generality, seems to have shaken Professor 
Pictet in his belief in the immutability of species. He who 
is acquainted with the distribution of existing species over 
the globe, will not attempt to account for the close resem- 
blance of distinct species in closely consecutive formations, 
by the physical conditions of the ancient areas having re- 
mained nearly the same. Let it be remembered that the 
forms of life, at least those inhabiting the sea, have changed 
almost simultaneously throughout the world, and therefore 
'under the most different climates and conditions. Consider 
the prodigious vicissitudes of climate during the pleistocene 
period, which includes the whole glacial epoch, and note how 
little the specific forms of the inhabitants of the sea have 
been affected. 

On the theory of descent, the full meaning of the fossil 
remains from closely consecutive formations being closely 
related, though ranked as distinct species, is obvious. As 
the accumulation of each formation has often been inter- 
rupted, and as long blank intervals have intervened between 
successive formations, we ought not to expect to find, as I 
attempted to show in the last chapter, in any one or in any 
two formations, all the intermediate varieties between the 
species which appeared at the commencement and close of 
these periods : but we ought to find after intervals, very long 
as measured by years, but only moderately long as measured 
geologically, closely allied forms, or, as they have been called 
by some authors, representative species; and these assuredly 
we do find. We find, in short, such evidence of the slow 
and scarcely sensible mutations of specific forms, as we have 
the right to expect. 


We have seen in the fourth chapter that the degree of 
differentiation and specialisation of the parts in organic 
beings, when arrived at maturity, is the best standard, as yet 
suggested, of their degree of perfection or highness. We 
have also seen that, as the specialisation of parts is an ad- 
vantage to each being, so natural selection will tend to render 


the organisation of each being more specialised and perfect, 
and in this sense higher; not but that it may leave many 
creatures with simple and unimproved structures fitted for 
simple conditions of life, and in some cases will even de- 
grade or simplify the organisation, yet leaving such degraded 
beings better fitted for their new walks of life. In another 
and more general manner, new species become superior to 
their predecessors; for they have to beat in the struggle for 
life all the older forms, with which they come into close 
competition. We may therefore conclude that if under a 
nearly similar climate the eocene inhabitants of the world 
could be put into competition with the existing inhabitants, 
the former would be beaten and exterminated by the latter, 
as would the secondary by the eocene, and the palaeozoic by 
the secondary forms. So that by this fundamental test of 
victory in the battle for life, as well as by the standard of 
the specialisation of organs, modern forms ought, on the 
theory of natural selection, to stand higher than ancient 
forms. Is this the case? A large majority of palaeon- 
tologists would answer in the affirmative; and it seems 
that this answer must be admitted as true, though difficult 
of proof. 

It is no valid objection to this conclusion, that certain 
Brachiopods have been but slightly modified from an ex- 
tremely remote geological epoch; and that certain land and 
fresh-water shells have remained nearly the same, from the 
time when, as far as is known, they first appeared. It is not 
an insuperable difficulty that Foraminifera have not, as in- 
sisted on by Dr. Carpenter, progressed in organisation since 
even the I.aurentian epoch ; for some organisms would have 
to remain fitted for simple conditions of life, and what could 
be better fitted for this end than these lowly organised Pro- 
tozoa? Such objections as the above would be fatal to my 
view, if it included advance in organisation as a necessary 
contingent. They would likewise be fatal, if the above Fora- 
minifera, for instance, could be proved to have first come 
into existence during the Laurentian epoch, or the above 
Brachiopods during the Cambrian formation ; for in this 
case, there would not have been time sufficient for the de- 
velopment of these organisms up to the standard which they 



had then reached. When advanced up to any given point, 
there is no necessity, on the theory of natural selection, for 
) their further continued progress; though they will, during 
each successive age, have to be slightly modified, so as to 
hold their places in relation to slight changes in their condi- 
tions. The foregoing objections hinge on the question 
whether we really know how old the world is, and at what 
period the various forms of life first appeared; and this may 
well be disputed. 

The problem whether organisation on the whole has ad- 
vanced is in many ways excessively intricate. The geological 
record, at all times imperfect, does not extend far enough 
back, to show with unmistakeable clearness that within the 
known history of the world organisation has largely ad- 
vanced. Even at the present day, looking to members of the 
same class, naturalists are not unanimous which forms ought 
to be ranked as highest: thus, some look at the selaceans or 
sharks, from their approach in some important points of 
structure to reptiles, as the highest fish; others look at the 
teleosteans as the highest. The ganoids stand intermediate 
between the selaceans and teleosteans; the latter at the 
present day are largely preponderant in number; but for- 
merly selaceans and ganoids alone existed; and in this case, 
according to the standard of highness chosen, so will it be 
said that fishes have advanced or retrograded in organisa- 
tion. To attempt to compare members of distinct types in 
the scale of highness seems hopeless ; who will decide whether 
a cuttle-fish be higher than a bee — that insect which the 
great Von Baer believed to be "in fact more highly organised 
than a fish, although upon another type"? In the complex 
struggle for life it is quite credible that crustaceans, not very 
high in their own class, might beat cephalopods, the highest 
molluscs; and such crustaceans, though not highly developed, 
would stand very high in the scale of invertebrate animals, if 
judged by the most decisive of all trials — the law of battle. 
Beside these inherent difficulties in deciding which forms 
are the most advanced in organisation, we ought not solely 
to compare the highest members of a class at any two 
periods — though undoubtedly this is one and perhaps the 
most important element in striking a balance — but we ought 


to compare all the members, high and low, at the two periods. 
At an ancient epoch the highest and lowest molluscoidal ani- 
mals, namely, cephalopods and brachiopods, swarmed in 
numbers; at the present time both groups are greatly re- 
duced, whilst others, intermediate in organisation, have 
largely increased; consequently some naturalists maintain 
that molluscs were formerly more highly developed than at 
present; but a stronger case can be made out on the oppo- 
site side, by considering the vast reduction of the brachio- 
pods, and the fact that our existing cephalopods, though few 
in number, are more highly organised than their ancient rep- 
resentatives. We ought also to compare the relative propor- 
tional numbers at any two periods of the high and low classes 
throughout the world: if, for instance, at the present day 
fifty thousand kinds of vertebrate animals exist, and if we 
knew that at some former period only ten thousand kinds 
existed, we ought to look at this increase in number in the 
highest class, which implies a great displacement of lower 
forms, as a decided advance in the organisation of the world. 
We thus see how hopelessly difficult it is to compare with 
perfect fairness under such extremely complex relations, the 
standard of organisation of the imperfectly-known faunas 
of successive periods. 

We shall appreciate this difficulty more clearly, by looking 
to certain existing faunas and floras. From the extraordi- 
nary manner in which European productions have recently 
spread over New Zealand, and have seized on places which 
must have been previously occupied by the indigenes, we 
must believe, that if all the animals and plants of Great 
Britain were set free in New Zealand, a multitude of British 
forms would in the course of time become thoroughly nat- 
uralised there, and would exterminate many of the natives. 
On the other hand, from the fact that hardly a single inhabi- 
tant of the southern hemisphere has become wild in any part 
of Europe, we may well doubt whether, if all the productions 
of New Zealand were set free in Great Britain, any consid- 
erable number would be enabled to seize on places now occu- 
pied by our native plants and animals. Under this point of 
view, the productions of Great Britain stand much higher in 
the scale than those of New Zealand. Yet the most skilful 


naturalist, from an examination of the species of the two 
countries, could not have foreseen this result. 

Agassiz and several other highly competent judges insist 
that ancient animals resemble to a certain extent the em- 
bryos of recent animals belonging to the same classes ; and 
that the geological succession of extinct forms is nearly par- 
allel with the embryological development of existing forms. 
This view accords admirably well with our theory. In a 
future chapter I shall attempt to show that the adult differs 
from its embryo, owing to variations having supervened at a 
not early age, and having been inherited at a corresponding 
age. This process, whilst it leaves the embryo almost unal- 
tered, continually adds, in the course of successive genera- 
tions, more and more difference to the adult. Thus the 
embryo comes to be left as a sort of picture, preserved by 
nature, of the former and less modified condition of the 
species. This view may be true, and yet may never be 
capable of proof. Seeing, for instance, that the oldest known 
mammals, reptiles, and fishes strictly belong to their proper 
classes, though some of these old forms are in a slight de- 
gree less distinct from each other than are the typical mem- 
bers of the same groups at the present day, it would be vain 
to look for animals having the common embryological char- 
acter of the Vertebrata, until beds rich in fossils are discov- 
ered far beneath the lowest Cambrian strata — a discovery of 
which the chance is small. 


Mr. Clift many years ago showed that the fossil mammals 
from the Australian caves were closely allied to the living 
marsupials of that continent. In South America, a similar 
relationship is manifest, even to an uneducated eye, in the 
gigantic pieces of armour, like those of the armadillo, found 
in several parts of La Plata ; and Professor Owen has shown 
in the most striking manner that most of the fossil mammals, 
buried there in such numbers, are related to South American 
types. This relationship is even more clearly seen in the 
wonderful collection of fossil bones made by MM. Lund and 


Clausen in the caves of Brazil. I was so much impressed 
with these facts that I strongly insisted, in 1839 and 1845, 
on this "law of the succession of types," — on "this won- 
derful relationship in the same continent between the dead 
and the living." Professor Owen has subsequently extended 
the same generalisation to the mammals of the Old World. 
We see the same law in this author's restorations of the 
extinct and gigantic birds of New Zealand. We see it also 
in the birds of the caves of Brazil. Mr. Woodward has 
shown that the same law holds good with sea-shells, but, 
from the wide distribution of most molluscs, it is not well 
displayed by them. Other cases could be added, as the rela- 
tion between the extinct and living land-shells of Madeira; 
and between the extinct and living brackish water-shells of 
the Aralo-Caspian Sea. 

Now what does this remarkable law of the succession of 
the same types within the same areas mean? He would be 
a bold man who, after comparing the present climate of Aus- 
tralia and of parts of South America, under the same lati- 
tude, would attempt to account, on the one hand through 
dissimilar physical conditions, for the dissimilarity of the 
inhabitants of these two continents; and, on the other hand 
through similarity of conditions, for the uniformity of the 
same types in each continent during the later tertiary periods. 
Nor can it be pretended that it is an immutable law that 
marsupials should have been chiefly or solely produced in 
Australia; or that Edentata and other American types should 
have been solely produced in South America. For we know 
that Europe in ancient times was peopled by numerous mar- 
supials; and I have shown in the publications above alluded 
to, that in America the law of distribution of terrestrial 
mammals was formerly different from what it now is. North 
America formerly partook strongly of the present character 
of the southern half of the continent; and the southern half • 
was formerly more closely allied, than it is at present, to the 
northern half. In a similar manner we know, from Falconer 
and Cautley's discoveries, that Northern India was formerly- 
more closely related in its mammals to Africa than it is at 
the present time. Analogous facts could be given in rela- 
tion to the distribution of marine animals. 


On the theory of descent with modification, the great law 
of the long enduring, but not immutable, succession of the 
same types within the same areas, is at once explained; for 
the inhabitants of each quarter of the world will obviously 
tend to leave in that quarter, during the next succeeding 
period of time, closely allied though in some degree modified 
descendants. If the inhabitants of one continent formerly 
differed greatly from those of another continent, so will 
their modified descendants still differ in nearly the same 
manner and degree. But after very long intervals of time, 
and after great geographical changes, permitting much inter- 
migration, the feebler will yield to the more dominant forms, 
and there will be nothing immutable in the distribution of 
organic beings. 

It may be asked in ridicule, whether I suppose that the 
megatherium and other allied huge monsters, which formerly 
lived in South America, have left behind them the sloth, 
armadillo, and anteater, as their degenerate descendants. 
This cannot for an instant be admitted. These huge animals 
have become wholly extinct, and have left no progeny. But 
in the caves of Brazil, there are many extinct species which 
are closely allied in size and in all other characters to the 
species still living in South America; and some of these 
fossils may have been the actual progenitors of the living 

It must not be forgotten that, on our theory, all the 
species of the same genus are the descendants of some one 
species; so that, if six genera, each having eight species, be 
found in one geological formation, and in a succeeding 
formation there be six other allied or representative genera 
each with the same number of species, then we may con- 
clude that generally only one species of each of the older 
genera has left modified descendants, which constitute the 
new genera containing the several species; the other seven 
species of each old genus having died out and left no progeny. 
Or, and this will be a far commoner case, two or three spe- 
cies in two or three alone of the six older genera will be 
the parents of the new genera : the other species and the other 
old genera having become utterly extinct. In failing orders, 
with the genera and species decreasing in numbers as is the 


case with the Edentata of South America, still fewer genera 
and species will leave modified blood-descendants. 


I have attempted to show that the geological record is ex- 
tremely imperfect; that only a small portion of the globe has 
been geologically explored with care; that only certain 
classes of organic beings have been largely preserved in a 
fossil state; that the number both of specimens and of spe- 
cies, preserved in our museums, is absolutely as nothing com- 
pared with the number of generations which must have 
passed away even during a single formation; that, owing to 
subsidence being almost necessary for the accumulation of 
deposits rich in fossil species of many kinds, and thick enough 
to outlast future degradation, great intervals of time must 
have elapsed between most of our successive formations; 
that there has probably been more extinction during the 
periods of subsidence, and more variation during the periods 
of elevation, and during the latter the record will have been 
least perfectly kept; that each single formation has not been 
continuously deposited; that the duration of each formation 
is probably short compared with the average duration of 
specific forms; that migration has played an important part 
in the first appearance of new forms in any one area and 
formation ; that widely ranging species are those which have 
varied most frequently, and have oftenest given rise to new 
species; that varieties have at first been local; and lastly, 
although each species must have passed through numerous 
transitional stages, it is probable that the periods, during 
which each underwent modification, though many and long 
as measured by years, have been short in comparison with 
the periods during which each remained in an unchanged 
condition. These causes, taken conjointly, will to a large ex- 
tent explain why — though we do find many links — we do not 
find interminable varieties, connecting together all extinct 
and existing forms by the finest graduated steps. It should 
also be constantly borne in mind that any linking variety 
between two forms, which might be found, would be ranked, 
unless the whole chain could be perfectly restored, as a new 


and distinct species; for it is not pretended that we have 
any sure criterion by which species and varieties can be 

He who rejects this view of the imperfection of the geo- 
logical record, will rightly reject the whole theory. For he 
may ask in vain where are the nmnberless transitional links 
which must formerly have connected the closely allied or 
representative species, found in the successive stages of the 
same great formation? He may disbelieve in the immense 
intervals of time which must have elapsed between our con- 
secutive formations ; he may overlook how important a part 
migration has played, when the formations of any one great 
region, as those of Europe, are considered; he may urge the 
apparent, but often falsely apparent, sudden coming in of 
whole groups of species. He may ask where are the remains 
of those infinitely numerous organisms which must have ex- 
isted long before the Cambrian system was deposited? We 
now know that at least one animal did then exist; but I can 
answer this last question only by supposing that where our 
oceans now extend they have extended for an enormous 
period, and where our oscillating continents now stand they 
have stood since the commencement of the Cambrian system; 
but that, long before that epoch, the world presented a widely 
different aspect; and that the older continents, formed of 
formations older than any known to us, exist now only as 
remnants in a metamorphosed condition, or lie still buried 
under the ocean. 

Passing from these difficulties, the other great leading 
facts in palaeontology agree admirably with the theory of 
descent with modification through variation and natural 
selection. We can thus understand how it is that new spe- 
cies come in slowly and successively; how species of dif- 
ferent classes do not necessarily change together, or at the 
same rate, or in the same degree ; yet in the long run that all 
undergo modification to some extent. The extinction of old 
forms is the almost inevitable consequence of the production 
of new forms. We can understand why, when a species has 
once disappeared, it never reappears. Groups of species in- 
crease in numbers slowly, and endure for unequal periods 
of time; for the process of modification is necessarily slow, 


and depends on many complex contingencies. The dominant 
species belonging to large and dominant groups tend to leave 
many modified descendants, which form new sub-groups and 
groups. As these are formed, the species of the less vig- 
orous groups, from their inferiority inherited from a com- 
mon progenitor, tend to become extinct together, and to leave 
no modified offspring on the face of the earth. But the utter 
extinction of a whole group of species has sometimes been a 
slow process, from the survival of a few descendants, lin- 
gering in protected and isolated situations. When a group 
has once wholly disappeared, it does not reappear; for the 
link of generation has been broken. 

We can understand how it is that dominant forms which 
spread widely and yield the greatest number of varieties tend 
to people the world with allied, but modified, descendants; 
and these will generally succeed in displacing the groups 
which are their inferiors in the struggle for existence. 
Hence, after long intervals of time, the productions of the 
world appear to have changed simultaneously. 

We can understand how it is that all the forms of life, 
ancient and recent, make together a few grand classes. We 
can understand, from the continued tendency to divergence 
of character, why the more ancient a form is, the more it 
generally differs from those now living; why ancient and 
extinct forms often tend to fill up gaps between existing 
forms, sometimes blending two groups, previously classed 
as distinct, into one; but more commonly bringing them only 
a little closer together. The more ancient a form is, the 
more often it stands in some degree intermediate between 
groups now distinct ; for the more ancient a form is, the 
more nearly it will be related to, and consequently resemble, 
the common progenitor of groups, since become widely 
divergent. Extinct forms are seldom directly intermediate 
between existing forms ; but are intermediate only by a long 
and circuitous course through other extinct and different 
forms. We can clearly see why the organic remains of 
closely consecutive formations are closely allied; for they 
are closely linked together by generation. We can clearly 
see why the remains of an intermediate formation arc inter- 
mediate in character. 


The inhabitants of the world at each successive period in 
its histor> have beaten their predecessors in the race for 
life, and are, in so far, higher in the scale, and their struc- 
ture has generally become more specialised; and this may 
account foi the common belief lield by so many palaeontolo- 
gists, that organisation on the whole has progressed. Extinct 
and ancient animals resemble to a certain extent the embryos 
of the more recent animals belonging to the same classes, 
and this wonderful fact receives a simple explanation accord- 
ing to our views. The succession of the same types of 
structure within the same areas during the later geological 
periods ceases to be mysterious, and is intelligible on the 
principle of inheritance. 

If then the geological record be as imperfect as many be- 
lieve, and it may at least be asserted that the record cannot 
be proved to be much more perfect, the main objections to 
the theory of natural selection are greatly diminished or dis- 
appear. On the other hand, all the chief laws of palaeontology 
plainly proclaim, as it seems to me, that species have been 
produced by ordinary generation : old forms having been sup- 
planted by new and improved forms of life, the products of 
Variation and the Survival of the Fittest. 

Geographical Distribution 

Present distribution cannot be accounted for by differences in physi- 
cal conditions — Importance of barriers — Affinity of the produc- 
tions of the same continent — Centres of creation — Means of 
dispersal, by changes of climate and of the level of the land, and 
by occasional means — Dispersal during the Glacial period — - 
Alternate Glacial periods in the North and South. 

IN considering the distribution of organic beings over the 
face of the globe, the first great fact which strikes us is, 
that neither the similarity nor the dissimilarity of the 
inhabitants of various regions can be wholly accounted for by 
climatal and other physical conditions. Of late, almost every 
author who has studied the subject has come to this conclu- 
sion. The case of America alone would almost suffice to 
prove its truth; for if we exclude the arctic and northern 
temperate parts, all authors agree that one of the most fun- 
damental divisions in geographical distribution is that be- 
tween the New and Old Worlds; yet if we travel over the 
vast American continent, from the central parts of the 
United States to its extreme southern point, we meet with 
the most diversified conditions ; humid districts, arid deserts, 
lofty mountains, grassy plains, forests, marshes, lakes, and 
great rivers, under almost every temperature. There is 
hardly a climate or condition in the Old World which can- 
not be paralleled in the New — at least as closely as the same 
species generally require. No doubt small areas can be 
pointed out in the Old World hotter than any in the New 
World; but these are not inhabited by a fauna different from 
that of the surrounding districts; for it is rare to find a group 
of organisms confined to a small area, of which the con- 
ditions are peculiar in only a slight degree. Notwithstand- 
ing this general parallelism in the conditions of the Old and 
New Worlds, how widely different are their living pro- 
ductions ! 



In the southern hemisphere, if we compare large tracts of 
land in Australia, South Africa, and western South America, 
between latitudes 25° and 35°, we shall find parts extremely 
similar in all their conditions, yet it would not be possible to 
point out three faunas and floras more utterly dissimilar. 
Or, again, we may compare the productions of South Amer- 
ica south of lat. 35° with those north of 25°, which conse- 
quently are separated by a space of ten degrees of latitude, 
and are exposed to considerably different conditions ; yet they 
are incomparably more closely related to each other than 
they are to the productions of Australia or Africa under 
nearly the same climate. Analogous facts could be given 
with respect to the inhabitants of the sea. 

A second great fact which strikes us in our general review 
is, that barriers of any kind, or obstacles to free migration, 
are related in a close and important manner to the differ- 
ences between the productions of various regions. We see 
this in the great differences in nearly all the terrestrial pro- 
ductions of the New and Old Worlds, excepting in the 
northern parts, where the land almost joins, and where, under 
a slightly different climate, there might have been free mi- 
gration for the northern temperate forms, as there now is 
for the strictly arctic productions. We see the same fact in 
the great difference between the inhabitants of Australia, 
Africa, and South America under the same latitude; for 
these countries are almost as much isolated from each other 
as is possible. On each continent, also, we see the same 
fact; for on the opposite sides of lofty and continuous moun- 
tain-ranges, of great deserts and even of large rivers, we 
find different productions; though as mountain-chains, des- 
erts, &c., are not as impassable, or likely to have endured so 
long, as the oceans separating continents, the differences are 
very inferior in degree to those characteristic of distinct 

Turning to the sea, we find the same law. The marine 
inhabitants of the eastern and western shores of South 
America are very distinct, with extremely few shells, Crus- 
tacea, or echinodermata in common; but Dr. Giinther has 
recently shown that about thirty per cent, of the fishes are 
the same on the opposite sides of the isthmus of Panama; 


and this fact has led naturaHsts to believe that the isthmus 
was formerly open. Westward of the shores of America, a 
wide space of open ocean extends, with not an island as a 
haltinj^-place for emigrants ; here we have a barrier of an- 
other kind, and as soon as this is passed we meet in the east- 
ern islands of the Pacific with another and totally distinct 
fauna. So that three marine faunas range far northward 
and southward in parallel lines not far from each other, 
under corresponding climates ; but from being separated from 
each other by impassable barriers, either of land or open sea, 
they are almost wholly distinct. On the other hand, proceed- 
ing still farther westward from the eastern islands of the 
tropical parts of the Pacific, we encounter no impassable 
barriers, and we have innumerable islands as halting-places, 
or continuous coasts, until, after travelling over a hemisphere, 
we come to the shores of Africa; and over this vast space 
we meet with no well-defined and distinct marine faunas. 
Although so few marine animals are common to the above- 
named three approximate faunas of Eastern and Western 
America and the eastern Pacific islands, yet many fishes 
range from the Pacific into the Indian Ocean, and many 
shells are common to the eastern islands of the Pacific and 
the eastern shores of Africa on almost exactly opposite 
meridians of longitude. 

A third great fact, partly included in the foregoing state- 
ment, is the affinity of the productions of the same continent 
or of the same sea, though the species themselves are dis- 
tinct at dififerent points and stations. It is a law of the 
widest generality, and every continent offers innumerable 
instances. Nevertheless, the naturalist, in travelling, for 
instance, from north to south, never fails to be struck by 
the manner in which successive groups of beings, specifically 
distinct, though nearly related, replace each other. Pie hears 
from closely allied, yet distinct kinds of birds, notes nearly 
similar, and sees their nests similarly constructed, but not 
quite alike, with eggs coloured in nearly the same manner. 
The plains near the Straits of Magellan are inhabited by 
one species of Rhea (American ostrich), and northward the 
plains of La Plata by another species of the same genus ; and 
not by a true ostrich or emu, like those inhabiting Africa 


and Australia under the same latitude. On these same plains 
of La Plata we see the agouti and bizcacha, animals having 
nearly the same habits as our hares and rabbits, and belong- 
ing to the same order of Rodents, but they plainly display 
an American type of structure. We ascend the lofty peaks 
of the Cordillera, and we find an alpine species of bizcacha; 
we look to the waters, and we do not find the beaver or 
musk-rat, but the coypu and capybara, rodents of the S. 
American type. Innumerable other instances could be given. 
If we look to the islands off the American shore, however 
much they may differ in geological structure, the inhabitants 
are essentially American, though they may be all peculiar 
species. We may look back to past ages, as shown in the 
last chapter, and we find American types then prevailing dn 
the American continent and in the American seas. We see 
in these facts some deep organic bond, throughout space and 
time, over the same areas of land and water, independently 
of physical conditions. The naturalist must be dull who is 
not led to inquire what this bond is. 

The bond is simply inheritance, that cause which alone, 
as far as we positively know, produces organisms quite like 
each other, or, as we see in the case of varieties, nearly 
alike. The dissimilarity of the inhabitants of different re- 
gions may be attributed to modification through variation 
and natural selection, and probably in a subordinate degree 
to the definite influence of different physical conditions. The 
degrees of dissimilarity will depend on the migration of the 
more dominant forms of life from one region into another 
having been more or less effectually prevented, at periods 
more or less remote; — on the nature and number of the for- 
mer immigrants; — and on the action of the inhabitants on 
each other in leading to the preservation of different modifi- 
cations; the relation of organism to organism in the struggle 
for life being, as I have already often remarked, the most 
important of all relations. Thus the high importance of 
barriers comes into play by checking migration ; as does time 
for the slow process of modification through natural selec- 
tion. Widely-ranging species, abounding in individuals, 
which have already triumphed over many competitors in 
their own widely-extended homes, will have the best chance 


of seizing on new places, when they spread into new coun- 
tries. In their new homes they will be exposed to new con- 
ditions, and will frequently undergo further modification and 
improvement; and thus they will become still further vic- 
torious, and will produce groups of modified descendants. 
Or. this principle of inheritance with modification we can 
understand how it is that sections of genera, whole genera, 
and even families, are confined to the same areas, as is so 
commonly and notoriously the case. 

There is no evidence, as was remarked in the last chapter, 
of the existence of any law of necessary development. As 
the variability of each species is an independent property, 
and will be taken advantage of by natural selection, only so 
far as it profits each individual in its complex struggle for 
life, so the amount of modification in different species will 
be no uniform quantity. If a number of species, after hav- 
ing long competed with each other in their old home, were 
to migrate in a body into a new and afterwards isolated 
country, they would be little liable to modification ; for 
neither migration nor isolation in themselves effect anything. 
These principles come into play only by bringing organisms 
into new relations with each other and in a lesser degree 
with the surrounding physical conditions. As we have seen 
in the last chapter that some forms have retained nearly the 
same character from an enormously remote geological period, 
so certain species have migrated over vast spaces, and have 
not become greatly or at all modified. 

According to these views, it is obvious that the several 
species of the same genus, though inhabiting the most dis- 
tant quarters of the world, must originally have proceeded 
from the same source, as they are descended from the same 
progenitor. In the case of those species which have under- 
gone during whole geological periods little modification, 
there is not much difficulty in believing that they have mi- 
grated from the same region ; for during the vast geographi- 
cal and climatal changes which have supervened since ancient 
times, almost any amount of migration is possible. But in 
many other cases, in which we have reason to believe that 
the species of a genus have been produced within compara- 
tively recent times, there is great difficulty on this head. It 


is also obvious that the individuals of the same species, 
though now inhabiting distant and isolated regions, must have 
proceeded from one spot, where their parents were first pro- 
duced : for, as has been explained, it is incredible that indi- 
viduals identically the same should have been produced from 
parents specifically distinct. 

Single Centres of supposed Creation. — We are thus 
brought to the question which has been largely discussed by 
naturalists, namely, whether species have been created at 
one or more points of the earth's surface. Undoubtedly 
there are many cases of extreme difficulty in understanding 
how the same species could possibly have migrated from 
some one point to the several distant and isolated points, 
where now found. Nevertheless the simplicity of the view 
that each species was first produced within a single region 
captivates the mind. He who rejects it, rejects the vera 
causa of ordinary generation with subsequent migration, and 
calls in the agency of a miracle. It is universally admitted, 
that in most cases the area inhabited by a species is con- 
tinuous ; and that when a plant or animal inhabits two points 
so distant from each other, or with an interval of such a 
nature, that the space could not have been easily passed over 
by migration, the fact is given as something remarkable and 
exceptional. The incapacity of migrating across a wide sea 
is more clear in the case of terrestrial mammals than perhaps 
with any other organic beings; and, accordingly, we find no 
inexplicable instances of the same mammals inhabiting dis- 
tant points of the world. No geologist feels any difficulty in 
Great Britain possessing the same quadrupeds with the rest 
of Europe, for they were no doubt once united. But if the 
same species can be produced at two separate points, why do 
we not find a single mammal common to Europe and Aus- 
tralia or South America? The conditions of life are nearly 
the same, so that a multitude of European animals and plants 
have become naturalised in America and Australia; and 
some of the aboriginal plants are identically the same at 
these distant points of the northern and southern hemi- 
spheres. The answer, as I believe, is, that mammals have 
not been able to migrate, whereas some plants, from their 
varied means of dispersal, have migrated across the wide and 


broken interspaces. The great and striking influence of bar- 
riers of all kinds, is intelligible only on the view that the 
great majority of species have been produced on one side, 
and have not been able to migrate to the opposite side. 
Some few families, many sub-families, very many genera, 
and a still greater number of sections of genera, are con- 
fined to a single region; and it has been observed by several 
naturalists that the most natural genera, or those genera in 
which the species are most closely related to each other, are 
generally confined to the same country, or if they have a 
wide range that their range is continuous. What a strange 
anomaly it would be, if a directly opposite rule were to pre- 
vail, when we go down one step lower in the series, namely, 
to the individuals of the same species, and these had not 
been, at least at first, confined to some one region! 

Hence it seems to me, as it has to many other naturalists, 
that the view of each species having been produced in one 
area alone, and having subsequently migrated from that area 
as far as its powers of migration and subsistence under past 
and present conditions permitted, is the most probable. Un- 
doubtedly many cases occur, in which we cannot explain how 
the same species could have passed from one point to the 
other. But the geographical and climatal changes which 
have certainly occurred within recent geological times, must 
have rendered discontinuous the formerly continuous range 
of many species. So that we are reduced to consider whether 
the exceptions to continuity of range are so numerous and 
of so grave a nature, that we ought to give up the belief, 
rendered probable by general considerations, that each species 
has been produced within one area, and has migrated thence 
as far as it could. It would be hopelessly tedious to discuss 
all the exceptional cases of the same species, now living at 
distant and separated points, nor do I for a moment pretend 
that any explanation could be offered of many instances. 
But, after some preliminary remarks, I will discuss a few of 
the most striking classes of facts; namely, the existence of 
the same species on the summits of distant mountain ranges, 
and at distant points in the arctic and antarctic regions; and 
secondly (in the following chapter), the wide distribution of 
fresh-water productions; and thirdly, the occurrence of the 


same terrestrial species on islands and on the nearest main- 
land, though separated by hundreds of miles of open sea. 
If the existence of the same species at distant and isolated 
points of the earth's surface, can in many instances be ex- 
plained on the view of each species having migrated from a 
single birthplace; then, considering our ignorance with re- 
spect to former climatal and geographical changes and to 
the various occasional means of transport, the belief that a 
single birthplace is the law, seems to me incomparably the 

In discussing this subject, we shall be enabled at the same 
time to consider a point equally important for us, namely, 
whether the several species of a genus which must on our 
theory all be descended from a common progenitor, can have 
migrated, undergoing modification during their migration, 
from some one area. If, when most of the species inhabiting 
one region are different from those of another region, though 
closely allied to them, it can be shown that migration from 
the one region to the other has probably occurred at some 
former period, our general view will be much strengthened* 
for the explanation is obvious on the principle of descent 
with modification. A volcanic island, for instance, upheaved 
and formed at the distance of a few hundreds of miles from 
a continent, would probably receive from it in the course of 
time a few colonists, and their descendants, though modified, 
would still be related by inheritance to the inhabitants of 
that continent. Cases of this nature are common, and are, 
as we shall hereafter see, inexplicable on the theory of inde- 
pendent creation. This view of the relation of the species 
of one region to those of another, does not differ much from 
that advanced by Mr. Wallace, who concludes that "every 
species has come into existence coincident both in space and 
time with a pre-existing closely allied species." And it is 
now well known that he attributes this coincidence to descent 
with modification. 

The question of single or multiple centres of creation dif- 
fers from another though allied question, — namely, whether 
all the individuals of the same species are descended from a 
single pair, or single hermaphrodite, or wtiether, as some 
authors suppose, from many individuals simultaneously ere- 


ated. With organic beings which never intercross, if such 
exist, each species must be descended from a succession of 
modified varieties, that have supplanted each other, but have 
never blended with other individuals or varieties of the same 
species; so that, at each successive stage of modification, all 
the individuals of the same form will be descended from a 
single parent. But in the great majority of cases, namely, 
with all organisms which habitually unite for each birth, or 
which occasionally intercross, the individuals of the same 
species inhabiting the same area will be kept nearly uniform 
by intercrossing; so that many individuals will go on simul- 
taneously changing, and the whole amount of modification at 
each stage will not be due to descent from a single parent. 
To illustrate what I mean: our English race-horses dififer 
from the horses of every other breed; but they do not owe 
their difference and superiority to descent from any single 
pair, but to continued care in the selecting and training of 
many individuals during each generation. 

Before discussing the three classes of facts, which I have 
selected as presenting the greatest amount of difficulty on the 
theory of "single centres of creation," I must say a few 
words on the means of dispersal. 


Sir C. Lyell and other authors have ably treated this sub- 
ject. I can give here only the briefest abstract of the more 
important facts. Change of climate must have had a power- 
ful influence on migration. A region now impassable to cer- 
tain organisms from the nature of its climate, might have 
been a high road for migration, when the climate was dif- 
ferent. I shall, however, presently have to discuss this 
branch of the subject in some detail. Changes of level in 
the land must also have been highly influential: a narrow 
isthmus now separates two marine faunas; submerge it, or 
let it formerly have been submerged, and the two faunas 
will now blend together, or may formerly have blended. 
Where the sea now extends, land may at a former period 
have connected islands or possibly even continents together, 
and thus have allowed terrestrial productions to pass from 


one to the other. No geologist disputes that great muta- 
tions of level have occurred within the period of existing 
organisms. Edward Forbes insisted that all the islands in 
the Atlantic must have been recently connected with Europe 
or Africa, and Europe likewise with America. Other authors 
have thus hypothetically bridged over every ocean, and 
united almost every island with some mainland. If indeed 
the arguments used by Forbes are to be trusted, it must be 
admitted that scarcely a single island exists which has not 
recently been united to some continent. This view cuts the 
Gordian knot of the dispersal of the same species to the most 
distant points, and removes many a difficulty; but to the best 
of my judgment we are not authorised in admitting such 
enormous geographical changes within the period of existing 
species. It seems to me that we have abundant evidence of 
great oscillations in the level of the land or sea; but not of 
such vast changes in the position and extension of our con- 
tinents, as to have united them within the recent period to 
each other and to the several intervening oceanic islands. 
I freely admit the former existence of many islands, now 
buried beneath the sea, which may have served as halting- 
places for plants and for many animals during their migra- 
tion. In the coral-producing oceans such sunken islands are 
now marked by rings of coral or atolls standing over them. 
Whenever it is fully admitted, as it will some day be, that 
each species has proceeded from a single birthplace, and 
when in the course of time we know something definite about 
the means of distribution, we shall be enabled to speculate 
with security on the former extension of the land. But I do 
not believe that it will ever be proved that within the recent 
period most of our continents which now stand quite sep- 
arate, have been continuously, or almost continuously united 
with each other, and with the many existing oceanic islands. 
Several facts in distribution — such as the great difference in 
the marine faunas on the opposite sides of almost every con- 
tinent, — the close relation of the tertiary inhabitants of sev- 
eral lands and even seas to their present inhabitants, — the 
degree of affinity between the mammals inhabiting islands 
with those of the nearest continent, being in part determined 
(as we shall hereafter see) by the depth of the intervening 


ocean, — these and other such facts are opposed to the admis- 
sion of such prodigious geographical revolutions within the 
recent period, as arc necessary on the view advanced by 
Forbes and admitted by his followers. The nature and rela- 
tive proportions of the inhabitants of oceanic islands are 
likewise opposed to the belief of their former continuity with 
continents. Nor does the almost universally volcanic com- 
position of such islands favour the admission that they are 
the wrecks of sunken continents; — if they had originally 
existed as continental mountain ranges, some at least of the 
islands would have been formed, like other mountain sum- 
mits, of granite, mctamorphic schists, old fossiliferous and 
other rocks, instead of consisting of mere piles of volcanic 

I must now say a few words on what are called accidental 
means, but which more properly should be called occasional 
means of distribution. I shall here confine myself to plants. 
In botanical works, this or that plant is often stated to be ill 
adapted for wide dissemination ; but the greater or less facili- 
ties for transport across the sea may be said to be almost 
wholly unknown. Until I tried, with Mr. Berkeley's aid. a 
few experiments, it was not even known how far seeds 
could resist the injurious action of sea-water. To my sur- 
prise I found that out of 87 kinds, 64 germinated after an 
immersion of 28 days, and a few survived an immersion of 
137 davs. It deserves notice that certain orders were far 
more injured than others: nine Leguminosae were tried, and, 
with one exception, they resisted the salt-water badly ; seven 
species of the allied orders, Hydrophyllacede and Polemo- 
niacere, were all killed by a month's immersion. For con- 
venience' sake I chiefly tried small seeds without the cap- 
sule or fruit; and as all of these sank in a few days they 
could not have been floated across wide spaces of the sea, 
whether or not they were injured by the salt-water. After- 
wards I tried some larger fruits, capsules, &c., and some of 
these floated for a long time. It is well known what a dif- 
ference there is in the buoyancy of green and seasoned tim- 
ber ; and it occurred to me that floods would often wash into 
the sea dried plants or branches with seed-capsules or fruit 
attached to them. Hence I was led to dry the stems and 


branches of 94 plants with ripe fruit, and to place them on 
sea-water. The majority sank quickly, but some which, 
whilst green, floated for a very short time, when dried floated 
much longer ; for instance, ripe hazel-nuts sank immediately, 
but when dried they floated for 90 days, and afterwards when 
planted germinated; an asparagus-plant with ripe berries 
floated for 23 days, when dried it floated for 85 days, and 
the seeds afterwards germinated ; the ripe seeds of Helosci- 
adium sank in two days, when dried they floated for above 
90 days, and afterwards germinated. Altogether, out of the 
94 dried plants, 18 floated for above 28 days ; and some of 
the 18 floated for a very much longer period. So that as |^ 
kinds of seeds germinated after an immersion of 28 days; 
and as -^f distinct species with ripe fruit (but not all the same 
species as in the foregoing experiment) floated, after being 
dried, for above 28 days, we may conclude, as far as anything 
can be inferred from these scanty facts, that the seeds of -^^-^ 
kinds of plants of any country might be floated by sea-cur- 
rents during 28 days, and would retain their power of ger- 
mination. In Johnston's Physical Atlas, the average rate of 
the several Atlantic currents is 33 miles per diem (some cur- 
rents running at the rate of 60 miles per diem) ; on this 
average, the seeds of ^-^ plants belonging to one country 
might be floated across 924 miles of sea to another country, 
and when stranded, if blown by an inland gale to a favour- 
able spot, would germinate. 

Subsequently to my experiments, M. Martens tried similar 
ones, but in a much better manner, for he placed the seeds 
in a box in the actual sea, so that they were alternately wet 
and exposed to the air like really floating plants. He tried 
98 seeds, mostly different from mine; but he chose many 
large fruits and likewise seeds from plants which live near 
the sea: and this would have favoured both the average 
length of their flotation and their resistance to the injurious 
action of the salt-water. On the other hand, he did not pre- 
viously dry the plants or branches with the fruit; and this, 
as we have seen, would have caused some of them to have 
floated much longer. The result was that -^f of his seeds of 
different kinds floated for 42 days, and were then capable of 
germination. But I do not doubt that plants exposed to the 


waves would float for a less time than those protected from 
violent movement as in our experiments. Therefore it would 
perhaps be safer to assume that the seeds of about ^% plants 
of a flora, after having been dried, could be floated across a 
space of sea 900 miles in width, and would then germinate. 
The fact of the larger fruits often floating longer than the 
small, is interesting; as plants with large seeds or fruit which, 
as Alph. de Candolle has shown, generally have restricted 
ranges, could hardly be transported by any other means. 

Seeds may be occasionally transported in another manner. 
Drift timber is thrown up on most islands, even on those in 
the midst of the widest oceans ; and the natives of the coral- 
islands in the Pacific procure stones for their tools, solely 
from the roots of drifted trees, these stones being a valuable 
royal tax. I find that when irregularly shaped stones are 
embedded in the roots of trees, small parcels of earth are fre- 
quently enclosed in their interstices and behind them, — so 
perfectly that not a particle could be washed away during the 
longest transport: out of one small portion of earth thus 
completely enclosed by the roots of an oak about 50 years 
old, three dicotyledonous plants germinated; I am certain of 
the accuracy of this observation. Again, I can show that 
the carcases of birds, when floating on the sea, sometimes 
escape being immediately devoured : and many kinds of seeds 
in the crops of floating birds long retain their vitality: peas 
and vetches, for instance, are killed by even a few days' im- 
mersion in sea-water; but some taken out of the crop of a 
pigeon, which had floated on artificial sea-water for 30 days, 
to my surprise nearly all germinated. 

Living birds can hardly fail to be highly effective agents 
in the transportation of seeds. I cpuld give many facts 
showing how frequently birds of many kinds are blown by 
gales to vast distances across the ocean. We may safely 
assume that under such circumstances their rate of flight 
would often be 25 miles an hour; and some authors have 
given a far higher estimate. I have never seen an instance 
of nutritious seeds passing through the intestines of a bird; 
but hard seeds of fruit pass uninjured through even the di- 
gestive organs of a turkey. In the course of two months, I 
picked up in my garden 12 kinds of seeds, out of the excre- 


ment of small birds, and these seemed perfect, and some of 
them, which were tried, germinated. But the following fact 
is more important: the crops of birds do not secrete gastric 
juice, and do not, as I know by trial, injure in the least the 
germination of seeds; now, after a bird has found and de- 
voured a large supply of food, it is positively asserted that 
all the grains do not pass into the gizzard for twelve or even 
eighteen hours. A bird in this interval might easily be 
blown to the distance of 500 miles, and hawks are known to 
look out for tired birds, and the contents of their torn crops 
might thus readily get scattered. Some hawks and owls 
bolt their prey whole, and, after an interval of from twelve 
to twenty hours, disgorge pellets, which, as I know from 
experiments made in the Zoological Gardens, include seeds 
capable of germination. Some seeds of the oat, wheat, mil- 
let, canary, hemp, clover, and beet germinated after having 
been from twelve to twenty-one hours in the stomachs of 
different birds of prey ; and two seeds of beet grew after hav- 
ing been thus retained for two days and fourteen hours. 
Fresh-water fish, I find, eat seeds of many land and water 
plants; fish are frequently devoured by birds, and thus the 
seeds might be transported from place to place. I forced 
many kinds of seeds into the stomachs of dead fish, and then 
gave their bodies to fishing-eagles, storks, and pelicans; 
these birds, after an interval of many hours, either rejected 
the seeds in pellets or passed them in their excrement; and 
several of these seeds retained the power of germination. 
Certain seeds, however, were always killed by this process. 
Locusts are sometimes blown to great distances from the 
land ; I myself caught one 370 miles from the coast of Africa, 
and have heard of others caught at greater distances. The 
Rev. R. T. Lowe informed Sir C. Lyell that in November 
1844 swarms of locusts visted the island of Madeira. They 
were in countless numbers, as thick as the flakes of snow in 
the heaviest snowstorm, and extended upwards as far as 
could be seen with a telescope. During two or three days 
they slowly careered round and round in an immense ellipse, 
at least five or six miles in diameter, and at night alighted 
on the taller trees, which were completely coated with them. 
They then disappeared over the sea, as suddenly as they had 


appeared, and have not since visited the island. Now, in 
parts of Natal it is believed by some farmers, though on in- 
sufficient evidence, that injurious seeds are introduced into 
their grassland in the dung left by the great flights of locusts 
v^'hich often visit that country. In consequence of this be- 
lief Mr. Weale sent me in a letter a small packet of the dried 
pellets, out of which I extracted under the microscope several 
seeds, and raised from them seven grass plants, belonging to 
two species, of two genera. Hence a swarm of locusts, such 
as that which visited Madeira, might readily be the means of 
introducing several kinds of plants into an island lying far 
from the mainland. 

Although the beaks and feet of birds are generally clean, 
earth sometimes adheres to them : in one case I removed 
sixty-one grains, and in another case twenty-two grains of 
dry argillaceous earth from the foot of a partridge, and in 
the earth there was a pebble as large as the seed of a vetch. 
Here is a better case : the leg of a woodcock was sent to me 
by a friend, with a little cake of dry eartli attached to the 
shank, weighing only nine grains ; and this contained a seed 
of the toad-rush (Juncus bufonius) which germinated and 
flowered. Mr. Swaysland, of Brighton, who during the last 
forty years has paid close attention to our migratory birds, 
informs me that he has often shot wagtails (Motacillae), 
wheatears, and whincats (Saxicolse), on their first arrival 
on our shores, before they had alighted ; and he has several 
times noticed little cakes of earth attached to their feet. 
Many facts could be given showing how generally soil is 
charged with seeds. For instance. Prof. Newton sent nie 
the leg of a red-legged partridge (Caccabis rufa) which had 
been wounded and could not fly, with a ball of hard earth 
adhering to it, and weighing six and a half ounces. The 
earth had been kept for three years, but when broken, 
watered and placed under a bell-glass, no less than 82 plants 
sprung from it: these consisted of 12 monocotyledons, includ- 
ing the common oat, and at least one kind of grass, and of 70 
dicotyledons, which consisted, judging from the young leaves, 
of at least three distinct species. With such facts before us, 
can we doubt that the many birds which are annually blown 
by gales across great spaces of ocean, and which annually 


migrate— for instance, the millions of quails across the Medi- 
terranean — must occasionally transport a few seeds embedded 
in dirt adhering to their feet or beaks? But I shall have to 
recur to this subject. 

As icebergs are known to be sometimes loaded with earth 
and stones, and have even carried brushwood, bones, and the 
nest of a land-bird, it can hardly be doubted that they must 
occasionally, as suggested by Lyell, have transported seeds 
form one part to another of the arctic and antarctic regions ; 
and during the Glacial period from one part of the now tem- 
perate regions to another. In the Azores, from the large 
number of plants common to Europe, in comparison with the 
species on the other islands of the Atlantic, which stand 
nearer to the mainland, and (as remarked by Mr. H. C. 
Watson) from their somewhat northern character in com- 
parison with the latitude, I suspected that these islands had 
been partly stocked by ice-borne seeds, during the Glacial 
epoch. At my request Sir C. Lyell wrote to M. Hartung 
to inquire whether he had observed erratic boulders on these 
islands, and he answered that he had found large fragments 
of granite and other rocks, which do not occur in the archi- 
pelago. Hence we may safely infer that icebergs formerly 
landed their rocky burthens on the shores of these mid-ocean 
islands, and it is at least possible that they may have brought 
thither some few seeds of northern plants. 

Considering that these several means of transport, and that 
other means, which without doubt remain to be discovered, 
have been in action year after year for tens of thousands of 
years, it would, I think, be a marvellous fact if many plants 
had not thus become widely transported. These means of 
transport are sometimes called accidental, but this is not 
strictly correct: the currents of the sea are not accidental, 
nor is the direction of prevalent gales of wind. It should be 
observed that scarcely any means of transport would carry 
seeds for very great distances: for seeds do not retain their 
vitality when exposed for a great length of time to the action 
of sea-water; nor could they be long carried in the crops or 
intestines of birds. These means, however, would suffice for 
occasional transport across tracts of sea some hundred miles 
in breadth, or from island to island, or from a continent to a 


neighbouring island, but not from one distant continent to 
another. The floras of distant continents would not by such 
means become mingled ; but would remain as distinct as they 
now are. The currents, from their course, would never 
bring seeds from North America to Britain, though they 
might and do bring seeds from the West Indies to our west- 
ern shores, where, if not killed by their long immersion in 
salt-water, they could not endure our climate. Almost every 
year, one or two land-birds are blown across the whole At- 
lantic Ocean, from North America to the western shores of 
Ireland and England; but seeds could be transported by these 
rare wanderers only by one means, namely, by dirt adhering 
to their feet or beaks, which is in itself a rare accident. 
Even in this case, how small would be the chance of a seed 
falling on favourable soil, and coming to maturity ! But it 
would be a great error to argue that because a well-stocked 
island, like Great Britain, has not, as far as is known (and 
it would be very difficult to prove this), received within the 
last few centuries, through occasional means of transport, 
immigrants from Europe or any other continent, that a 
poorly-stocked island, though standing more remote from the 
mainland, would not receive colonists by similar means. Out 
of a hundred kinds of seeds or animals transported to an 
island, even if far less well-stocked than Britain, perhaps 
not more than one would be so well fitted to its new home, 
as to become naturalised. But this is no valid argument 
against what would be effected by occasional means of trans- 
port, during the long lapse of geological time, whilst the 
island was being upheaved, and before it had become fully 
stocked with inhabitants. On almost bare land, with few or 
no destructive insects or birds living there, nearly every seed 
which chanced to arrive, if fitted for the climate, would ger- 
minate and survive. 


The identity of many plants and animals, on mountain- 
summits, separated from each other by hundreds of miles of 
lowlands, where Alpine species could not possibly exist, is 
one of the most striking cases known of the same species 


living at distant points, without the apparent possibility of 
their having migrated from one point to the other. It is in- 
deed a remarkable fact to see so many plants of the same 
species living on the snowy regions of the Alps or Pyrenees, 
and in the extreme northern parts of Europe; but it is far 
more remarkable, that the plants on the White Mountains, 
in the United States of America, are all the same with those 
of Labrador, and nearly all the same, as we hear from Asa 
Gray, with those on the loftiest mountains of Europe. Even 
as long ago as 1747, such facts led Gmelin to conclude that 
the same species must have been independently created at 
many distinct points ; and we might have remained in this 
same belief, had not Agassiz and others called vivid atten- 
tion to the Glacial period, which, as we shall immediately 
see, affords a simple explanation of these facts. We have 
evidence of almost every conceivable kind, organic and in- 
organic, that, within a very recent geological period, central 
Europe and North America suffered under an arctic climate. 
The ruins of a house burnt by fire do not tell their tale more 
plainly than do the mountains of Scotland and Wales, with 
their scored flanks, polished surfaces, and perched boulders, 
of the icy streams with which their valleys were lately filled. 
So greatly has the climate of Europe changed, that in North- 
ern Italy, gigantic moraines, left by old glaciers, are now 
clothed by the vine and maize. Throughout a large part of 
the United States, erratic boulders and scored rocks plainly 
reveal a former cold period. 

The former influence of the glacial climate on the distribu- 
tion of the inhabitants of Europe, as explained by Edward 
Forbes, is substantially as follows. But we shall follow the 
changes more readily, by supposing a new glacial period 
slowly to come on, and then pass away, as formerly occurred. 
As the cold came on, and as each more southern zone be- 
came fitted for the inhabitants of the north, these would take 
the places of the former inhabitants of the temperate regions. 
The latter, at the same time, would travel further and fur- 
ther southward, unless they were stopped by barriers, in 
which case they would perish. The mountains would become 
covered with snow and ice, and their former Alpine inhabit- 
ants would descend to the plains. By the time that the cold 


had reached its maximum, wc should have an arctic fauna 
and flora, covering the central parts of Europe, as far south 
as the Alps and Pyrenees, and even stretching into Spain. 
The now temperate regions of the United States would like- 
wise be covered by arctic plants and animals and these would 
be nearly the same with those of Europe ; for the present 
circumpolar inhabitants, which we suppose to have every- 
where travelled southward, are remarkably uniform round 
the world. 

As the warmth returned, the arctic forms would retreat 
northward, closely followed up in their retreat by the produc- 
tions of the more temperate regions. And as the snow 
melted from the bases of the mountains, the arctic forms 
would seize on the cleared and thawed ground, always as- 
cending, as the warmth increased and the snow still further 
disappeared, higher and higher, whilst their brethren were 
pursuing their northern journey. Hence, when the warmth 
had fully returned, the same species, which had lately lived 
together on the European and North American lowlands, 
would again be found in the arctic regions of the Old and 
New Worlds, and on many isolated mountain-summits far 
distant from each other. 

Thus we can understand the identity of many plants at 
points so immensely remote as the mountains of the United 
States and those of Europe. We can thus also understand 
the fact that the Alpine plants of each mountain-range are 
more especially related to the arctic forms living due north 
or nearly due north of them : for the first migration when 
the cold came on, and the re-migration on the returning 
warmth, would generally have been due south and north. 
The Alpine plants, for example, of Scotland, as remarked 
by Mr. H. C. Watson, and those of the Pyrenees, as re- 
marked by Ramond, are more especially allied to the plants 
of northern Scandinavia ; those of the United States to Lab- 
rador; those of the mountains of Siberia to the arctic regions 
of that country. These views, grounded as they are on the 
perfectly well-ascertained occurrence of a former Glacial 
period, seem to me to explain in so satisfactory a manner 
the present distribution of the Alpine and Arctic productions 
of Europe and America, that when in other regions we find 


the same species on distant mountain-summits, we may al- 
most conclude, without other evidence, that a colder climate 
formerly permitted their migration across the intervening 
lowlands, now become too warm for their existence. 

As the arctic forms moved first southward and afterwards 
backwards to the north, in unison with the changing climate, 
they will not have been exposed during their long migrations 
to any great diversity of temperature; and as they all mi- 
grated in a body together, their mutual relations will not 
have been much disturbed. Hence, in accordance with the 
principles inculcated in this volume, these forms will not have 
been liable to much modification. But with the Alpine pro- 
ductions, left isolated from the moment of the returning 
warmth, first at the bases and ultimately on the summits of 
the mountains, the case will have been somewhat different; 
for it is not likely that all the same arctic species will have 
been left on mountain-ranges far distant from each other, 
and have survived there ever since ; they will also in all prob- 
ability, have become mingled with ancient Alpine species, 
which must have existed on the mountains before the com- 
mencement of the Glacial epoch, and which during the cold- 
est period will have been temporarily driven down to the 
plains ; they will, also, have been subsequently exposed to 
somewhat different climatal influences. Their mutual rela- 
tions will thus have been in some degree disturbed; conse- 
quently they will have been liable to modification; and they 
have been modified ; for if we compare the present Alpine 
plants and animals of the several great European mountain- 
ranges one with another, though many of the species remain 
identically the same, some exist as varieties, some as doubt- 
ful forms or sub-species, and some as distinct yet closely 
allied species representing each other on the several ranges. 

In the foregoing illustration I have assumed that at the 
commencement of our imaginary Glacial period, the arctic 
productions were as uniform round the polar regions as they 
are at the present day. But it is also necessary to assume 
that many sub-arctic and some few temperate forms were 
the same round the world, for some of the species which 
now exist on the lower mountain-slopes and on the plains of 
North America and Europe are the same; and it may be 


asked how I account for this degree of uniformity in the 
sub-arctic and temperate forms round the world, at the com- 
mencement of the real Glacial period. At the present day, 
the sub-arctic and northern temperate productions of the Old 
and New Worlds are separated from each other by the whole 
Atlantic Ocean and by the northern part of the Pacific. 
During the Glacial period, when the inhabitants of the Old 
and New Worlds lived farther southwards than they do at 
present, they must have been still more completely separated 
from each other by wider spaces of ocean; so that it may 
well be asked how the same species could then or previously 
have entered the two continents. The explanation, I believe, 
lies in the nature of the climate before the commencement of 
the Glacial period. At this, the newer Pliocene period, the 
majority of the inhabitants of the world were specifically the 
same as now, and we have good reason to believe that the 
climate was warmer than at the present day. Hence we 
may suppose that the organisms which now live under lati- 
tude 60°. lived during the Pliocene period farther north 
under the Polar Circle, in latitude 66°-6y* ; and that the 
present arctic productions then lived on the broken land still 
nearer to the pole. Now, if we look at a terrestrial globe, 
we see under the Polar Circle that there is almost continuous 
land from western Europe, through Siberia, to eastern Amer- 
ica. And this continuity of the circumpolar land, with the 
consequent freedom under a more favourable climate for 
intermigration, will account for the supposed uniformity of 
the sub-arctic and temperate productions of the Old and New 
Worlds, at a period anterior to the Glacial epoch. 

Believing, from reasons before alluded to, that our conti- 
nents have long remained in nearly the same relative posi- 
tion, though subjected to great oscillations of level, I am 
strongly inclined to extend the above view, and to infer that 
during some still earlier and still warmer period, such as the 
older Pliocene period, a large number of the same plants and 
animals inhabited the almost continuous circumpolar land; 
and that these plants and animals, both in the Old and New 
Worlds, began slowly to migrate southwards as the climate 
became less warm, long before the commencement of the 
Glacial period. We now see, as I believe, their descendants, 


mostly in a modified condition, in the central parts of Europe 
and the United States. On this view we can understand the 
relationship with very little identity, between the productions 
of North America and Europe, — a relationship which is 
highly remarkable, considering the distance of the two areas, 
and their separation by the whole Atlantic Ocean. We can 
further understand the singular fact remarked on by several 
observers that the productions of Europe and America dur- 
ing the later tertiary stages were more closely related to 
each other than they are at the present time; for during 
these warmer periods the northern parts of the Old and New 
Worlds will have been almost continuously united by land, 
serving as a bridge, since rendered impassable by cold, for 
the intermigration of their inhabitants. 

During the slowly decreasing warmth of the Pliocene 
period, as soon as the species in common, which inhabited 
the New and Old Worlds, migrated south of the Polar 
Circle, they will have been completely cut off from each 
other. This separation, as far as the more temperate produc- 
tions are concerned, must have taken place long ages ago. 
As the plants and animals migrated southward, they will 
have become mingled in the one great region with the native 
American productions, and would have had to compete with 
them; and in the other great region, with those of the Old 
World. Consequently we have here everything favourable 
for much modification, — for far more modification than with 
the Alpine productions, left isolated, within a much more 
recent period, on the several mountain-ranges and on the 
arctic lands of Europe and N. America. Hence it has come, 
that when we compare the now living productions of the tem- 
perate regions of the New and Old Worlds, we find very few 
identical species (though Asa Gray has lately shown that 
more plants are identical than was formerly supposed), but 
we find in every great class many forms, which some nat- 
uralists rank as geographical races, and others as distinct 
species ; and a host of closely allied or representative forms 
which are ranked by all naturalists as specifically distinct. 

As on the land, so in the waters of the sea, a slow south- 
ern migration of a marine fauna, which, during the Pliocene 
or even a somewhat earlier period, was nearly uniform along 


the continuous shores of the Polar Circle, will account, on 
the theory of modification, for many closely allied forms now 
living in marine areas completely sundered. Thus, I think, 
we can understand the presence of some closely allied, still 
existing and extinct tertiary forms, on the eastern and west- 
ern shores of temperate North America; and the still more 
striking fact of many closely allied crustaceans (as described 
in Dana's admirable work), some fish and other marine ani- 
mals, inhabiting the Mediterranean and the seas of Japan, — 
these two areas being now completely separated by the 
breadth of a whole continent and by wide spaces of ocean. 

These cases of close relationship in species either now or 
formerly inhabiting the seas on the eastern and western 
shores of North America, the Mediterranean and Japan, and 
the temperate lands of North America and Europe, are inex- 
plicable on the theory of creation. We cannot maintain that 
such species have been created alike, in correspondence with 
the nearly similar physical conditions of the areas; for if we 
compare, for instance, certain parts of South America with 
parts of South Africa or Australia, we see countries closely 
similar in all their physical conditions, with their inhabitants 
utterly dissimilar. 


But we must return to our more immediate subject. I am 
convinced that Forbes' view may be largely extended. In 
Europe we meet with the plainest evidence of the Glacial 
period, from the western shores of Britain to the Oural range, 
and southward to the Pyrenees. We may infer from the 
frozen mammals and nature of the mountain vegetation, that 
Siberia was similarly affected. In the Lebanon, according 
to Dr. Hooker, perpetual snow formerly covered the central 
axis, and fed glaciers which rolled 4000 feet down the val- 
leys. The same observer has recently found great moraines 
at a low level on the Atlas range in N. Africa. Along the 
Himalaya, at points 900 miles apart, glaciers have left the 
marks of their former low descent; and in Sikkim. Dr. 
Hooker saw maize growing on ancient and gigantic moraines. 
Southward of the Asiatic continent, on the opposite side of 



the equator, we know, from the excellent researches of Dr. 
J. Haast and Dr. Hector, that in New Zealand immense 
glaciers formerly descended to a low level; and the same 
plants found by Dr. Hooker on widely separated mountains 
in this island tell the same story of a former cold period. 
From facts communicated to me by the Rev. W. B. Clarke, 
it appears also that there are traces of former glacial action 
on the mountains of the south-eastern corner of Australia. 

Looking to America; in the northern half, ice-borne frag- 
ments of rock have been observed on the eastern side of the 
continent, as far south as lat. 36° -37°, and on the shores of 
the Pacific, where the climate is now so different, as far 
south as lat. 46°. Erratic boulders have, also, been noticed 
on the Rocky Mountains. In the Cordillera of South Amer- 
ica, nearly under the equator, glaciers once extended far be- 
low their present level. In Central Chile I examined a vast 
mound of detritus with great boulders, crossing the Portillo 
valley, which there can hardly be a doubt once formed a huge 
moraine; and Mr. D. Forbes informs me that he found in 
various parts of the Cordillera, from lat. 13° to 30° S., at 
about the height of 12,000 feet, deeply-furrowed rocks, re- 
sembling those with which he was familiar in Norway, and 
likewise great masses of detritus, including grooved pebbles. 
Along this whole space of the Cordillera true glaciers do not 
now exist even at much more considerable heights. Farther 
south on both sides of the continent, from lat. 41° to the 
southernmost extremity, we have the clearest evidence of 
former glacial action, in numerous immense boulders trans- 
ported far from their parent source. 

From these several facts, namely from the glacial action 
having extended all round the northern and southern hemi- 
spheres — from the period having been in a geological sense 
recent in both hemispheres — from its having lasted in both 
during a great length of time, as may be inferred from the 
amount of work effected — and lastly from glaciers having 
recently descended to a low level along the whole line of the 
Cordillera, it at one time appeared to me that we could not 
avoid the conclusion that the temperature of the whole world 
had been simultaneously lowered during the Glacial period. 
But now Mr. CroU, in a series of admirable memoirs, has 


attempted to show that a glacial condition of climate is the 
result of various physical causes, brought into operation by 
an increase in the eccentricity of the earth's orbit. All these 
causes tend towards the same end; but the most powerful 
appears to be the indirect influence of the eccentricity of the 
orbit upon oceanic currents. According to Mr. Croll, cold 
periods regularly recur every ten or fifteen thousand years; 
and these at long intervals are extremely severe, owing to 
certain contingencies, of which the most important, as Sir C. 
Lyell has shown, is the relative position of the land and 
water. Mr. Croll believes that the last great Glacial period 
occurred about 240,000 years ago, and endured with slight 
alterations of climate for about 160,000 years. With respect 
to more ancient Glacial periods, several geologists are con- 
vinced from direct evidence that such occurred during the 
Miocene and Eocene formations, not to mention still more 
ancient formations. But the most important result for us, 
arrived at by Mr. Croll, is that whenever the northern hemi- 
sphere passes through a cold period the temperature 
of the southern hemisphere is actually raised, with the win- 
ters rendered much milder, chiefly through changes in the 
direction of the ocean-currents. So conversely it will be 
with the northern hemisphere, whilst the southern passes 
through a Glacial period. This conclusion throws so much 
light on geographical distribution that I am strongly inclined 
to trust in it; but I will first give the facts, which demand 
an explanation. 

In South America, Dr. Hooker has shown that besides 
many closely allied species, between forty and fifty of the 
flowering plants of Tierra del Fuego, forming no inconsider- 
able part of its scanty flora, are common to North America 
and Europe, enormously remote as these areas in opposite 
hemispheres are from each other. On the lofty mountains 
of equatorial America a host of peculiar species belonging 
to European genera occur. On the Organ mountains of 
Brazil, some few temperate European, some Antarctic, and 
some Andean genera were found by Gardner, which do not 
exist in the low intervening hot countries. On the Silla of 
Caraccas, the illustrious Humboldt long ago found species 
belonging to genera characteristic of the Cordillera. 


In Africa, several forms characteristic of Europe and some 
few representatives of the flora of the Cape of Good Hope 
occur on the mountains of Abyssinia. At the Cape of Good 
Hope a very few European species, beHeved not to have been 
introduced by man, and on the mountains several representa- 
tive European forms are found, which have not been dis- 
covered in the intertropical parts of Africa. Dr. Hooker 
has also lately shown that several of the plants living on the 
upper parts of the lofty island of Fernando Po and on the 
neighbouring Cameroon mountains, in the Gulf of Guinea, 
are closely related to those on the mountains of Abyssinia, 
and likewise to those of temperate Europe. It now also 
appears, as I hear from Dr. Hooker, that some of these same 
temperate plants have been discovered by the Rev. R. T. 
Lowe on the mountains of the Cape Verde islands. This 
extension of the same temperate forms, almost under the 
equator, across the whole continent of Africa and to the 
mountains of the Cape Verde archipelago, is one of the most 
astonishing facts ever recorded in the distribution of plants. 

On the Himalaya, and on the isolated mountain-ranges of 
the peninsula of India, on the heights of Ceylon, and on the 
volcanic cones of Java, many plants occur, either identically 
the same or representing each other, and at the same time 
representing plants of Europe, not found in the intervening 
hot lowlands. A list of the genera of plants collected on 
the loftier peaks of Java, raises a picture of a collection made 
on a hillock in Europe ! Still more striking is the fact that 
peculiar Australian forms are represented by certain plants 
growing on the summits of the mountains of Borneo. Some 
of these Australian forms, as I hear from Dr. Hooker, ex- 
tend along the heights of the peninsula of Malacca, and are 
thinly scattered on the one hand over India, and on the other 
hand as far north as Japan. 

On the southern mountains of Australia, Dr. F. Miiller has 
discovered several European species; other species, not in- 
troduced by man, occur on the lowlands ; and a long list can 
be given, as I am informed by Dr. Hooker, of European 
genera, found in Australia, but not in the intermediate torrid 
regions. In the admirable 'Introduction to the Flora of New 
Zealand/ by Dr. Hooker, analogous and striking facts are 


given in regard to the plants of that large island. Hence we 
see that certain plants growing on the more lofty mountains 
of the tropics in all parts of the world, and on the temperate 
plains of the north and south, are either the same species or 
varieties of the same species. It should, however, be ob- 
served that these plants are not strictly arctic forms ; for, as 
Mr. H. C. Watson has remarked, "in receding from polar 
towards equatorial latitudes, the Alpine or mountain floras 
really become less and less Arctic." Besides these identical 
and closely allied forms, many species inhabiting the same 
widely sundered areas, belong to genera not now found in 
the intermediate tropical lowlands. 

These brief remarks apply to plants alone; but some few 
analogous facts could be given in regard to terrestrial ani- 
mals. In marine productions, similar cases likewise occur; 
as an example, I may quote a statement by the highest 
authority, Prof. Dana, that "it is certainly a wonderful fact 
that New Zealand should have a closer resemblance in its 
Crustacea to Great Britain, its antipode, than to any other 
part of the world." Sir J. Richardson, also, speaks of the re- 
appearance on the shores of New Zealand, Tasmania, &c., of 
northern forms of fish. Dr. Hooker informs me that 
twenty-five species of Algae are common to New Zealand 
and to Europe, but have not been found in the intermediate 
tropical seas. 

From the foregoing facts, namely, the presence of tem- 
perate forms on the highlands across the whole of equatorial 
Africa, and along the Peninsula of India, to Ceylon and the 
Malay Archipelago, and in a less well-marked manner across 
the wide expanse of tropical South America, it appears 
almost certain that at some former period, no doubt during 
the most severe part of a Glacial period, the lowlands of 
these great continents were everywhere tenanted under the 
equator by a considerable number of temperate forms. At 
this period the equatorial climate at the level of the sea was 
probably about the same with that now experienced at the 
height of from five to six thousand feet under the same lati- 
tude, or perhaps even rather cooler. During this, the coldest 
period, the lowlands under the equator must have been 
clothed with a mingled tropical and temperate vegetation, 


like that described by Hooker as growing luxuriantly at the 
height of from four to five thousand feet on the lower slopes 
of the Himalayas, but with perhaps a still greater prepon- 
derance of temperate forms. So again in the mountainous 
islands of Fernando Po, in the Gulf of Guinea, Mr. Mann 
found temperate European forms beginning to appear at the 
height of about five thousand feet. On the mountains of 
Panama, at the height of only two thousand feet, Dr. See- 
mann found the vegetation like that of Mexico, "with forms 
of the torrid zone harmoniously blended with those of the 

Now let us see whether Mr. Croll's conclusion that when 
the northern hemisphere suffered from the extreme cold of 
the great Glacial period, the southern hemisphere was actu- 
ally warmer, throws any clear light on the present apparently 
inexplicable distribution of various organisms in the tem- 
perate parts of both hemispheres, and on the mountains of 
the tropics. The Glacial period, as measured by years, must 
have been very long; and when we remember over what vast 
spaces some naturalised plants and animals have spread 
within a few centuries, this period will have been ample for 
any amount of migration. As the cold became more and 
more intense, we know that Arctic forms invaded the tem- 
perate regions; and, from the facts just given, there can 
hardly be a doubt that some of the more vigorous, dominant 
and widest-spreading temperate forms invaded the equa- 
torial lowlands. The inhabitants of these hot lowlands would 
at the same time have migrated to the tropical and sub- 
tropical regions of the south, for the southern hemisphere 
was at this period warmer. On the decline of the Glacial 
period, as both hemispheres gradually recovered their former 
temperatures, the northern temperate forms living on the 
lowlands under the equator, would have been driven to their 
former homes or have been destroyed, being replaced by the 
equatorial forms returning from the south. Some, however, 
of the northern temperate forms would almost certainly have 
ascended any adjoining high land, where, if sufficiently lofty, 
they would have long survived like the Arctic forms on the 
mountains of Europe. They might have survived, even if 
the climate was not perfectly fitted for them, for the change 


of temperature must have been very slow, and plants un- 
doubtedly possess a certain capacity for acclimatisation, as 
shown by their transmitting to their offspring different con- 
stitutional powers of resisting heat and cold. 

In the regular course of events the southern hemisphere 
would in its turn be subjected to a severe Glacial period, with 
the northern hemisphere rendered warmer; and then the 
southern temperate forms would invade the equatorial low- 
lands. The northern forms which had before been left on 
the mountains would now descend and mingle with the south- 
ern forms. These latter, when the warmth returned, would 
return to their former homes, leaving some few species on 
the mountains, and carrying southward with them some of 
the northern temperate forms which had descended from 
their mountain fastnesses. Thus, we should have some few 
species identically the same in the northern and southern 
temperate zones and on the mountains of the intermediate 
tropical regions. But the species left during a long time on 
these mountains, or in opposite hemispheres, would have to 
compete with many new forms and would be exposed to 
somewhat different physical conditions; hence they would 
be eminently liable to modification, and would generally now 
exist as varieties or as representative species; and this is the 
case. We must, also, bear in mind the occurrence in both 
hemispheres of former Glacial periods; for these will ac- 
count, in accordance with the same principles, for the many 
quite distinct species inhabiting the same widely separated 
areas, and belonging to genera not now found in the inter- 
mediate torrid zones. 

It is a remarkable fact strongly insisted on by Hooker in 
regard to America, and by Alph. de Candolle in regard to 
Australia, that many more identical or slightly modified spe- 
cies have migrated from the north to the south, than in a 
reversed direction. We see, however, a few southern forms 
on the mountains of Borneo and Abyssinia. I suspect that 
this preponderant migration from the north to the south is 
due to the greater extent of land in the north, and to the 
northern forms having existed in their own homes in greater 
numbers, and having consequently been advanced through 
natural selection and competition to a higher stage of per- 


fection, or dominating power, than the southern forms. And 
thus, when the two sets became commingled in the equatorial 
regions, during the alternations of the Glacial periods, the 
northern forms were the more powerful and were able to 
hold their places on the mountains, and afterwards to mi- 
grate southward with the southern forms ; but not so the 
southern in regard to the northern forms. In the same 
manner at the present day, we see that very many European 
productions cover the ground in La Plata, New Zealand, and 
to a lesser degree in Australia, and have beaten the natives; 
whereas extremely few southern forms have become natu- 
ralised in any part of the northern hemisphere, though hides, 
wool, and other objects likely to carry seeds have been 
largely imported into Europe during the last two or three 
centuries from La Plata and during the last forty or fifty 
years from Australia. The Neilgherrie mountains in India, 
however, offer a partial exception ; for here, as I hear from 
Dr. Hooker, Australian forms are rapidly sowing themselves 
and becoming naturalised. Before the last great Glacial 
period, no doubt the intertropical mountains were stocked 
with endemic Alpine forms; but these have almost every- 
where yielded to the more dominant forms generated in the 
larger areas and more efficient workshops of the north. In 
many islands the native productions are nearly equalled, or 
even outnumbered, by those which have become naturalised; 
and this is the first stage towards their extinction. Moun- 
tains are islands on the land, and their inhabitants have 
yielded to those produced within the larger areas of the 
north, just in the same way as the inhabitants of real islands 
have everywhere yielded and are still yielding to continental 
forms naturalised through man's agency. 

The same principles apply to the distribution of terrestrial 
animals and of marine productions, in the northern and 
southern temperate zones, and on the intertropical mountains. 
When, during the height of the Glacial period, the ocean- 
currents were widely different to what they now are, some 
of the inhabitants of the temperate seas might have reached 
the equator ; of these a few would perhaps at once be able to 
migrate southward, by keeping to the cooler currents, whilst 
others might remain and survive in the colder depths until 


the southern hemisphere was in its turn subjected to a glacial 
cHmate and permitted their further progress; in nearly the 
same manner as, according to Forbes, isolated spaces inhab- 
ited by Arctic productions exist to the present day in the 
deeper parts of the northern temperate seas. 

I am far from supposing that all the difficulties in regard 
to the distribution and affinities of the identical and allied 
species, which now live so widely separated in the north and 
south, and sometimes on the intermediate mountain-ranges, 
are removed on the views above given. The exact lines of 
migration cannot be indicated. We cannot say why certain 
species and not others have migrated ; why certain species 
have been modified and have given rise to new forms, whilst 
others have remained unaltered. We cannot hope to explain 
such facts, until we can say why one species and not another 
becomes naturalised by man's agency in a foreign land ; why 
one species ranges twice or thrice as far, and is twice or 
thrice as common, as another species within their own homes. 

Various special difficulties also remain to be solved ; for 
instance, the occurrence, as shown by Dr. Hooker, of the 
same plants at points so enormously remote as Kerguelen 
Land, New Zealand, and Fuegia; but icebergs, as suggested 
by Lyell, may have been concerned in their dispersal. The 
existence at these and other distant points of the southern 
hemisphere, of species, which, though distinct, belong to 
genera exclusively confined to the south, is a more remark- 
able case. Some of these species are so distinct, that we 
cannot suppose that there has been time since the commence- 
ment of the last Glacial period for their migration and sub- 
sequent modification to the necessary degree. The facts 
seem to indicate that distinct species belonging to the same 
genera have migrated in radiating lines from a common 
genera; and I am inclined to look in the southern, as in the 
northern hemisphere, to a former and warmer period, before 
the commencement of the last Glacial period, when the .Vnt- 
arctic lands, now covered with ice, supported a highly ^ 
peculiar and isolated flora. It may be suspected that before 
this flora was exterminated during the last Glacial epoch, a 
few forms had been already widely dispersed to various 
points of the southern hemisphere by occasional means of 


transport, and by the aid as halting-places, of now sunken 
islands. Thus the southern shores of America, Australia, 
and New Zealand may have become slightly tinted by the 
same peculiar forms of life. 

Sir C. Lyell in a striking passage has speculated, in lan- 
guage almost identical with mine, on the effects of great 
alterations of climate throughout the world on geographical 
distribution. And we have now seen that Mr. Croll's conclu- 
sion that successive Glacial periods in the one hemisphere 
coincide with warmer periods in the opposite hemisphere, 
together with the admission of the slow modification of spe- 
cies, explains a multitude of facts in the distribution of the 
same and of the allied forms of life in all parts of the globe. 
The living waters have flowed during one period from the 
north and during another from the south, and in both cases 
have reached the equator; but the stream of life has flowed 
with greater force from the north than in the opposite direc- 
tion, and has consequently more freely inundated the south. 
As the tide leaves its drift in horizontal lines, rising higher 
on the shores where the tide rises highest, so have the living 
waters left their living drift on our mountain summits, in a 
line gently rising from the Arctic lowlands to great altitude 
under the equator. The various beings thus left stranded 
may be compared with savage races of man, driven up and 
surviving in the mountain fastnesses of almost every land, 
which serves as a record, full of interest to us, of the former 
inhabitants of the surrounding lowlands. 

Geographical Distribution — contimied 

Distribution of fresh-water productions — On the inhabitants of 
oceanic islands — Absence of Batrachians and of terrestrial Mam- 
mals — On the relation of the inhabitants of islands to those of 
the nearest mainland — On colonisation from the nearest source 
with subsequent modification — Summary of the last and present 


AS lakes and river-systems are separated from each 
l\ Other by barriers of land, it might have been thought 
-*- -^ that fresh-water productions would not have ranged 
widely within the same country, and as the sea is apparently 
a still more formidable barrier, that they would never have 
extended to distant countries. But the case is exactly the re- 
verse. Not only have many fresh-water species, belonging 
to different classes, an enormous range, but allied species 
prevail in a remarkable manner throughout the world. When 
first collecting in the fresh waters of Brazil, I well remember 
feeling much surprise at the similarity of the fresh-water 
insects, shells, &c., and at the dissimilarity of the surroimd- 
ing terrestrial beings, compared with those of Britain. 

But the wide ranging power of fresh-water productions 
can, I think, in most cases be explained by their having be- 
come fitted, in a manner highly useful to them, for short 
and frequent migrations from pond to pond, or from stream 
to stream, within their own countries; and liability to wide 
dispersal would follow from this capacity as an almost neces- 
sary consequence. We can here consider only a few cases; 
of these, some of the most difficult to explain are presented 
by fish. It was formerly believed that the same fresh-water 
species never existed on two continents distant from each 
other. But Dr. Giinther has lately shown that the Galaxias 



attenuatus inhabits Tasmania, New Zealand, the Falkland 
Islands, and the mainland of South America. This is a won- 
derful case, and probably indicates dispersal from an Ant- 
arctic centre during a former warm period. This case, how- 
ever, is rendered in some degree less surprising by the spe- 
cies of this genus having the power of crossing by some 
unknown means considerable spaces of open ocean: thus 
there is one species common to New Zealand and to the 
Auckland Islands, though separated by a distance of about 
230 miles. On the same continent fresh-water fish often 
range widely, and as if capriciously; for in two adjoining 
river-systems some of the species may be the same, and some 
wholly different. 

It is probable that they are occasionally transported by 
what may be called accidental means. Thus fishes still alive 
are not very rarely dropped at distant points by whirlwinds; 
and it is known that the ova retain their vitality for a con- 
siderable time after removal from the water. Their dispersal 
may, however, be mainly attributed to changes in the level 
of the land within the recent period, causing rivers to flow 
into each other. Instances, also, could be given of this 
having occurred during floods, without any change of level. 
The wide difference of the fish on the opposite sides of most 
mountain-ranges, which are continuous, and which conse- 
quently must from an early period have completely prevented 
the inosculation of the river-system on the two sides, leads to 
the same conclusion. Some fresh-water fish belong to very 
ancient forms, and in such cases there will have been ample 
time for great geographical changes, and consequently time 
and means for much migration. Moreover Dr. Giinther has 
recently been led by several considerations to infer that with 
fishes the same forms have a long endurance. Salt-water 
fish can with care be slowly accustomed to live in fresh 
water; and, according to Valenciennes, there is hardly a 
single group of which all the members are confined to fresh 
water, so that a marine species belonging to a fresh-water 
group might travel far along the shores of the sea, and 
could, it is probable, become adapted without much difficulty 
to the fresh waters of a distant land. 

Some species of fresh-water shells have very wide ranges, 


and allied species which, on our theory, are descended from 
a common parent, and must have proceeded from a single 
source, prevail throughout the world. Their distribution at 
first perplexed mc much, as their ova are not likely to be 
transported by birds; and the ova, as well as the adults, are 
immediately killed by sea-water. I could not even under- 
stand how some naturalised species have spread rapidly 
throughout the same country. But two facts, which I have 
observed — and many others no doubt will be discovered — 
throw some light on this subject. When ducks suddenly 
emerge from a pond covered with duck-weed, I have twice 
seen these little plants adhering to their backs; and it has 
happened to me, in removing a little duck-weed from one 
aquarium to another, that I have unintentionally stocked the 
one with fresh-water shells from the other. But another 
agency is perhaps more effectual : I suspended the feet of a 
duck in an aquarium, where many ova of fresh-water shells 
were hatching; and I found that numbers of the extremely 
minute and just-hatched shells crawled on the feet, and clung 
to them so firmly that when taken out of the water they 
could not be jarred off, though at a somewhat more advanced 
age they would voluntarily drop off. These just-hatched 
molluscs, though aquatic in their nature, survived on the 
(luck's feet, in damp air, from twelve to twenty hours ; and 
ill this length of time a duck or heron might fly at least six 
or seven hundred miles, and if blown across the sea to an 
oceanic island, or to any other distant point, would be sure 
to alight on a pool or rivulet. Sir Charles Lyell informs me 
that a Dytiscus has been caught with an Ancylus (a fresh- 
water shell like a limpet) firmly adhering to it; and a water- 
beetle of the same family, a Colymbetes, once dew on board 
the 'Beagle,' when forty-five miles distant from the nearest 
land : how much farther it might have been blown by a 
favouring gale no one can tell. 

With respect to plants, it has long been known what enor- 
mous ranges many fresh-water, and even marsh species, 
have, both over continents and to the most remote oceanic 
islands. This is strikingly illustrated, according to Alph. de 
Candolle, in those large groups of terrestrial plants, which 
have very few aquatic members; for the latter seem immedi- 


ately to acquire, as if in consequence, a wide range. I think 
favourable means of dispersal explain this fact. I have be- 
fore mentioned that earth occasionally adheres in some quan- 
tity to the feet and beaks of birds. Wading birds, which 
frequent the muddy edges of ponds, if suddenly flushed, 
would be the most likely to have muddy feet. Birds of this 
order wander more than those of any other; and they are 
occasionally found on the most remote and barren islands 
of the open ocean; they would not be likely to alight on the 
surface of the sea, so that any dirt on their feet would not be 
washed off; and when gaining the land, they would be sure to 
fly to their natural fresh-water haunts. I do not believe that 
botanists are aware how charged the mud of ponds is with 
seeds; I have tried several little experiments, but will here 
give only the most striking case: I took in February three 
table-spoonfuls of mud from three different points, beneath 
water, on the edge of a little pond: this mud when dried 
weighed only 6}i ounces; I kept it covered up in my study 
for six months, pulling up and counting each plant as it 
grew; the plants were of many kinds, and were altogether 
537 in number; and yet the viscid mud was all contained in 
a breakfast cup ! Considering these facts, I think it would 
be an inexplicable circumstance if water-birds did not trans- 
port the seeds of fresh-water plants to unstocked ponds and 
streams, situated at very distant points. The same agency 
may have come into play with the eggs of some of the 
smaller fresh-water animals. 

Other and unknown agencies probably have also played a 
part. I have stated that fresh-water fish eat some kinds of 
seeds, though they reject many other kinds after having 
swallowed them; even small fish swallow seeds of moderate 
size, as of the yellow water-lily and Potamogeton. Herons 
and other birds, century after century, have gone on daily 
devouring fish; they then take flight and go to other waters, 
or are blown across the sea; and we have seen that seeds 
retain their power of germination, when rejected many hours 
afterwards in pellets or in the excrement. When I saw the 
great size of the seeds of that fine water-lily, the Nelumbium, 
and remembered Alph. de Candolle's remarks on the distribu- 
tion of this plant, I though that the means of its dispersal 


must remain inexplicable; but Audubon states that he found 
the seeds of the great southern water-lily (probably, accord- 
ing to Dr. Hooker, the Nelunibium luteum) in a heron's 
stomach. Now this bird must often have flown with its 
stomach thus well stocked to distant ponds, and then getting 
a hearty meal of fish, analogy makes me believe that it 
would have rejected the seeds in a pellet in a fit state for 

In considering these several means of distribution, it should 
be remembered that when a pond or stream is first formed, 
for instance, on a rising islet, it will be unoccupied; and a 
single seed or egg will have a good chance of succeeding. 
Although there will always be a struggle for life between 
the inhabitants of the same pond, however few in kind, yet 
as the number even in a well-stocked pond is small in com- 
parison with the number of species inhabiting an equal area 
of land, the competition between them will probably be less 
severe than between terrestrial species; consequently an in- 
truder from the waters of a foreign country would have a 
better chance of seizing on a new place, than in the case 
of terrestrial colonists. We should also remember that many 
fresh-water productions are low in the scale of nature, and 
we have reason to believe that such beings become modified 
more slowly than the high ; and this will give time for the 
migration of aquatic species. We should not forget the 
probability of many fresh-water forms having formerly 
ranged continuously over immense areas, and then having 
become extinct at intermediate points. But the wide distri- 
bution of fresh-water plants and of the lower animals, 
whether retaining the same identical form or in some degree 
modified, apparently depends in main part on the wide dis- 
persal of their seeds and eggs by animals, more especially by 
fresh-water birds, which have great powers of flight, and 
naturally travel from one piece of water to another. 


We now come to the last of the three classes of facts, 
which I have selected as presenting the greatest amount of 
difficulty with respect to distribution, on the view that not 


only all the individuals of the same species have migrated 
from some one area, but that allied species, although now 
inhabiting the most distant points, have proceeded from a 
single area, — the birthplace of their early progenitors. I 
have already given my reason for disbelieving in continental 
extensions within the period of existing species, on so enor- 
mous a scale that all the many islands of the several oceans 
were thus stocked with their present terrestrial inhabitants. 
This view removes many difficulties, but it does not accord 
with all the facts in regard to the productions of islands. In 
the following remarks I shall not confine myself to the mere 
question of dispersal, but shall consider some other cases 
bearing on the truth of the two theories of independent crea- 
tion and of descent with modification. 

The species of all kinds which inhabit oceanic islands are 
few in number compared with those on equal continental 
areas: Alph. de Candolle admits this for plants, and Wollas- 
ton for insects. New Zealand, for instance, with its lofty 
mountains and diversified stations, extending over 780 miles 
of latitude, together with the outlying islands of Auckland, 
Campbell and Chatham, contain altogether only 960 kinds of 
flowering plants ; if we compare this moderate number with 
the species which swarm over equal areas in South-Western 
Australia or at the Cape of Good Hope, we must admit that 
some cause, independently of different physical conditions, 
has given rise to so great a difference in number. Even the 
uniform county of Cambridge has 847 plants, and the little 
island of Anglesea 764, but a few ferns and a few intro- 
duced plants are included in these numbers, and the compari- 
son in some other respects is not quite fair. We have 
evidence that the barren island of Ascension aboriginally 
possessed less than half-a-dozen flowering plants; yet many 
species have now become naturalised on it, as they have in 
New Zealand and on every other oceanic island which can 
be named. 

In St. Helena there is reason to believe that the natu- 
ralised plants and animals have nearly or quite extermi- 
nated many native productions. He who admits the doctrine 
of the creation of each separate species, will have to admit 
that a sufficient number of the best adapted plants and ani- 


mals were not created for oceanic islands; for man has unin- 
tentionally stocked them far more fully and perfectly than 
did nature. 

Although in oceanic islands the species are few in number, 
the proportion of endemic kinds (i.e. those found nowhere 
else in the world) is often extremely large. If we compare, 
for instance, the number of endemic land-shells in Madeira, 
or of endemic birds in the Galapagos Archipelago, with the 
number found on any continent, and then compare the area 
of the island with that of the continent, we shall see that this 
is true. This fact might have been theoretically expected, 
for, as already explained, species occasionally arriving after 
long intervals of time in the new and isolated district, and 
having to compete with new associates, would be eminently 
liable to modification, and would often produce groups of 
modified descendants. But it by no means follows that, be- 
cause in an island nearly all the species of one class are 
peculiar, those of another class, or of another section of the 
same class, are peculiar ; and this difference seems to depend 
partly on the species which are not modified having immi- 
grated in a body, so that their mutual relations have not 
been much disturbed ; and partly on the frequent arrival of 
unmodified immigrants from the mother-country, with which 
the insular forms have intercrossed. It should be borne in 
mind that the offspring of such crosses would certainly gain 
in vigour ; so that even an occasional cross would produce 
more effect than might have been anticipated. I will give a 
few illustrations of the foregoing remarks : in the Galapagos 
Islands there arc 26 land-birds; of these 21 for perhaps 23) 
are peculiar, whereas of the 11 marine birds only 2 are 
peculiar ; and it is obvious that marine birds could arrive at 
these islands much more easily and frequently than land- 
birds. Bermuda, on the other hand, which lies at about the 
same distance from North America as the Galapagos Islands 
do from South America, and which has a very peculiar soil, 
does not possess a single endemic land-bird : and wo know 
from Mr. J. M. Jones' admirable account of Bermuda, that 
very many North American birds occasionally or even fre- 
quently visit this island. Almost every year, as I am In- 
formed by Mr. E. V. Harcourt. many European and .Xfrican 


birds are blown to Madeira; this island is inhabited by gcf 
kinds, of which one alone is peculiar, though very closely 
related to a European form ; and three or four other species 
are confined to this island and to the Canaries. So that the 
Islands of Bermuda and Madeira have been stocked from 
the neighbouring continents with birds, which for long ages 
have there struggled together, and have become mutually 
co-adapted. Hence when settled in their new homes, each 
kind will have been kept by the others to its proper place 
and habits, and will consequently have been but little liable 
to modification. Any tendency to modification will also have 
been checked by intercrossing with the unmodified immi- 
grants, often arriving from the mother-country. Madeira 
again is inhabited by a wonderful number of peculiar land- 
shells, whereas not one species of sea-shell is peculiar to its 
shores; now, though we do not know how sea-shells are dis- 
persed, yet we can see that their eggs or larvae, perhaps at- 
tached to seaweed or floating timber, or to the feet of wading- 
birds, might be transported across three or four hundred 
miles of open sea far more easily than land-shells. The dif- 
ferent orders of insects inhabiting Madeira present nearly 
parallel cases. 

Oceanic islands are sometimes deficient in animals of cer- 
tain whole classes, and their places are occupied by other 
classes; thus in the Galapagos Islands reptiles, and in New 
Zealand gigantic wingless birds, take, or recently took, the 
place of mammals. Although New Zealand is here spoken 
of as an oceanic island, it is in some degree doubtful whether 
it should be so ranked; it is of large size, and is not sep- 
arated from Australia by a profoundly deep sea; from its 
geological character and the direction of its mountain-ranges, 
the Rev. W. B. Clarke has lately maintained that this island, 
as well as New Caledonia, should be considered as appur- 
tenances of Australia. Turning to plants. Dr. Hooker has 
shown that in the Galapagos Islands the proportional num- 
bers of the dififerent orders are very different from what they 
are elsewhere. All such differences in number, and the ab- 
sence of certain whole groups of animals and plants, are gen- 
erally accounted for by supposed differences in the physical 
conditions of the islands; but this explanation is not a little 


doubtful. Facility of immigration seems to have been fully 
as important as the nature of the conditions. 

Many remarkable little facts could be given with respect 
to the inhabitants of oceanic islands. For instance, in cer- 
tain islands not tenanted by a single mammal, some of the 
endemic plants have beautifully hooked seeds; yet few rela- 
tions are more manifest than that hooks serve for the trans- 
portal of seeds in the wool or fur of quadrupeds. But a 
hooked seed might be carried to an island by other means; 
and the plant then becoming modified would form an endemic 
species, still retaining its hooks, which would form a useless 
appendage like the shrivelled wings under the soldered wing- 
covers of many insular beetles. Again, islands often possess 
trees or bushes belonging to orders which elsewhere include 
only herbaceous species ; now trees, as Alph. de Candolle 
has shown, generally have, whatever the cause may be, con- 
fined ranges. Hence trees would be little likely to reach dis- 
tant oceanic islands ; and an herbaceous plant, which had no 
chance of successfully competing with the many fully devel- 
oped trees growing on a continent, might, when established 
on an island, gain an advantage over other herbaceous plants 
by growing taller and taller and overtopping them. In this 
case, natural selection would tend to add to the stature of the 
plant, to whatever order it belonged, and thus first convert 
it into a bush and then into a tree. 


With respect to the absence of whole orders of animals on 
oceanic islands, Bory St. Vincent long ago remarked that 
Batrachians (frogs, toads, newts) are never found on any of 
the many islands with which the great oceans are studded. 
I have taken pains to verify this assertion, and have found 
it true, with the exception of New Zealand, New Caledonia, 
the Andaman Islands, and perhaps the Salomon Islands and 
the Seychelles. But I have already remarked that it is 
doubtful whether New Zealand and New Caledonia ought to 
be classed as oceanic islands; and this is still more doubtful 
with respect to the Andaman and Salomon groups and the 


Seychelles. This general absence of frogs, toads, and newts 
on so many true oceanic islands cannot be accounted for by 
their physical conditions : indeed it seems that islands are 
peculiarly fitted for these animals ; for frogs have been intro- 
duced into Madeira, the Azores, and Mauritius, and have 
multiplied so as to become a nuisance. But as these animals 
and their spavin are immediately killed (with the exception, 
as far as known, of one Indian species) by sea-water, there 
would be great difficulty in their transportal across the sea, 
and therefore we can see why they do not exist on strictly 
oceanic islands. But why, on the theory of creation, they 
should not have been created there, it would be very difficult 
to explain. 

Mammals offer another and similar case. I have carefully 
searched the oldest voyages, and have not found a single 
instance, free from doubt, of a terrestrial mammal (excluding 
domesticated animals kept by the natives) inhabiting an island 
situated above 300 miles from a continent or great continental 
island; and many islands situated at a much less distance are 
equally barren. The Falkland Islands, which are inhabited 
by a wolf-like fox, come nearest to an exception ; but this 
group cannot be considered as oceanic, as it lies on a bank 
in connection with the mainland at the distance of about 280 
miles ; moreover, icebergs formerly brought boulders to its 
western shores, and they may have formerly transported 
foxes, as now frequently happens in the arctic regions. Yet 
it cannot be said that small islands will not support at least 
small mammals, for they occur in many parts of the world 
on very small islands, when lying close to a continent ; and 
hardly an island can be named on which our smaller quadru- 
peds have not become naturalised and greatly multiplied. It 
cannot be said, on the ordinary view of creation, that there 
has not been time for the creation of mammals ; many vol- | 

canic islands are sufficiently ancient, as shown by the stu- 
pendous degradation which they have suffered, and by their 
tertiary strata : there has also been time for the production 
of endemic species belonging to other classes ; and on conti- 
nents it is known that new species of mammals appear and 
disappear at a quicker rate than other and lower animals. 
Although terrestrial mammals do not occur on oceanic 


islands, aerial mammals do occur on almost every island. 
New Zealand possesses two bats found nowhere else in the 
world: Norfolk Island, the Viti Archipelago, the Bonin 
Islands, the Caroline and Marianne Archipelagoes, and Mau- 
ritius, all possess their peculiar bats. Why, it may be asked, 
has the supposed creative force produced bats and no other 
mammals on remote islands? On my view this question can 
easily be answered ; for no terrestrial mammal can be trans- 
ported across a wide space of sea, but bats can fly across. 
Bats have been seen wandering by day far over the Atlantic 
Ocean ; and two North American species either regularly or 
occasionally visit Bermuda, at the distance of 600 miles from 
the mainland. I hear from Mr. Tomes, who has specially 
studied this family